Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't assign same value.

Eliminate duplicated code.

Don't forget to release a thread reference count,
replace _thr_ref_add() with _thr_find_thread(),
so reference count is no longer needed.

MFC after: 3 days

use rtprio_thread system call to get or set thread priority.

Pass correct parameter size.

Caching scheduling policy and priority in userland, a critical but baddly
written application is frequently changing thread priority for SCHED_OTHER
policy.

Use thr_setscheduler, thr_getscheduler and thr_setschedparam to implement
pthread functions.

Use kernel facilities to support real-time scheduling.

- Use same priority range returned by kernel's sched_get_priority_min()
and sched_get_priority_max() syscalls.
- Remove unused fields from structure pthread_attr.

WARNS level 4 cleanup.

Remove priority mutex code because it does not work correctly,
to make it work, turnstile like mechanism to support priority
propagating and other realtime scheduling options in kernel
should be available to userland mutex, for the moment, I just
want to make libthr be simple and efficient thread library.

Discussed with: deischen, julian

Import my recent 1:1 threading working. some features improved includes:
1. fast simple type mutex.
2. __thread tls works.
3. asynchronous cancellation works ( using signal ).
4. thread synchronization is fully based on umtx, mainly, condition
variable and other synchronization objects were rewritten by using
umtx directly. those objects can be shared between processes via
shared memory, it has to change ABI which does not happen yet.
5. default stack size is increased to 1M on 32 bits platform, 2M for
64 bits platform.
As the result, some mysql super-smack benchmarks show performance is
improved massivly.

Okayed by: jeff, mtm, rwatson, scottl

o Assertions to catch that stuff that shouldn't happen is not happening.
o In the rwlock code: move a duplicated check inside an if..else to after
the if...else clause.
o When initializing a static rwlock move the initialization check
inside the lock.
o In thr_setschedparam.c: When breaking out of the trylock...retry if busy
loop make sure to reset the mtx pointer to null if the mutex is nolonger
in a queue.

Make libthr async-signal-safe without costly signal masking. The guidlines I
followed are: Only 3 functions (pthread_cancel, pthread_setcancelstate,
pthread_setcanceltype) are required to be async-signal-safe by POSIX. None of
the rest of the pthread api is required to be async-signal-safe. This means
that only the three mentioned functions are safe to use from inside
signal handlers.
However, there are certain system/libc calls that are
cancellation points that a caller may call from within a signal handler,
and since they are cancellation points calls have to be made into libthr
to test for cancellation and exit the thread if necessary. So, the
cancellation test and thread exit code paths must be async-signal-safe
as well. A summary of the changes follows:

o Almost all of the code paths that masked signals, as well as locking the
pthread structure now lock only the pthread structure.
o Signals are masked (and left that way) as soon as a thread enters
pthread_exit().
o The active and dead threads locks now explicitly require that signals
are masked.
o Access to the isdead field of the pthread structure is protected by both
the active and dead list locks for writing. Either one is sufficient for
reading.
o The thread state and type fields have been combined into one three-state
switch to make it easier to read without requiring a lock. It doesn't need
a lock for writing (and therefore for reading either) because only the
current thread can write to it and it is an integer value.
o The thread state field of the pthread structure has been eliminated. It
was an unnecessary field that mostly duplicated the flags field, but
required additional locking that would make a lot more code paths require
signal masking. Any truly unique values (such as PS_DEAD) have been
reborn as separate members of the pthread structure.
o Since the mutex and condvar pthread functions are not async-signal-safe
there is no need to muck about with the wait queues when handling
a signal ...
o ... which also removes the need for wrapping signal handlers and sigaction(2).
o The condvar and mutex async-cancellation code had to be revised as a result
of some of these changes, which resulted in semi-unrelated changes which
would have been difficult to work on as a separate commit, so they are
included as well.

The only part of the changes I am worried about is related to locking for
the pthread joining fields. But, I will take a closer look at them once this
mega-patch is committed.

o Catch up with the mutex priority protocol fixes.
o Move pthread_getschedparam() into the same file with it's
pthread_set* counterpart. Copyright on both files is identical.

Comment out most of pthread_setschedparam. Pthread priorities didn't
work before anyways, and I didn't want to fix broken code I had no
way of testing. It was necessary however, in order to get rid of GIANT_LOCK.
Pthread priorities will have to wait a little longer to get fixed.

Make WARNS2 clean. The fixes mostly included:
o removed unused variables
o explicit inclusion of header files
o prototypes for externally defined functions

Approved by: re/blanket libthr

- Define curthread as _get_curthread() and remove all direct calls to
_get_curthread(). This is similar to the kernel's curthread. Doing
this saves stack overhead and is more convenient to the programmer.
- Pass the pointer to the newly created thread to _thread_init().
- Remove _get_curthread_slow().

- Reenable setschedparam and the prioceiling code now that the mutex code
has been restored.

- Add libthr but don't hook it up to the regular build yet. This is an
adaptation of libc_r for the thr system call interface. This is beta
quality code.