Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Simplify _get_curthread() and _tcb_ctor because libc and rtld now
already allocate thread pointer space in tls block for initial thread.
Only i386 and amd64 have been done, others still have to be tested.

Remove functions i386_get_gsbase and i386_set_gsbase, they were already
in libc.

Remove unused variable.

Now libthr only uses GDT based tls on i386. using LDT can only increase
clock cycles and has 8191 threads limitation.

Add i386_get_gsbase, i386_set_gsbase since old libc doesn't have the
functions, otherwise user ports have to be rebuilt.

Adapt the libpthread patch for using i386_set_gsbase() to libthr.

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