simplify pthread_attr_t stack/guard size representation

previously, the pthread_attr_t object was always initialized all-zero,
and stack/guard size were represented as differences versus their
defaults. this required lots of confusing offset arithmetic everywhere
they were used. instead, have pthread_attr_init fill in the default
values, and work with absolute sizes everywhere.

make futex operations use private-futex mode when possible

private-futex uses the virtual address of the futex int directly as
the hash key rather than requiring the kernel to resolve the address
to an underlying backing for the mapping in which it lies. for certain
usage patterns it improves performance significantly.

in many places, the code using futex __wake and __wait operations was
already passing a correct fixed zero or nonzero flag for the priv
argument, so no change was needed at the site of the call, only in the
__wake and __wait functions themselves. in other places, especially
where the process-shared attribute for a synchronization object was
not previously tracked, additional new code is needed. for mutexes,
the only place to store the flag is in the type field, so additional
bit masking logic is needed for accessing the type.

for non-process-shared condition variable broadcasts, the futex
requeue operation is unable to requeue from a private futex to a
process-shared one in the mutex structure, so requeue is simply
disabled in this case by waking all waiters.

for robust mutexes, the kernel always performs a non-private wake when
the owner dies. in order not to introduce a behavioral regression in
non-process-shared robust mutexes (when the owning thread dies), they
are simply forced to be treated as process-shared for now, giving
correct behavior at the expense of performance. this can be fixed by
adding explicit code to pthread_exit to do the right thing for
non-shared robust mutexes in userspace rather than relying on the
kernel to do it, and will be fixed in this way later.

since not all supported kernels have private futex support, the new
code detects EINVAL from the futex syscall and falls back to making
the call without the private flag. no attempt to cache the result is
made; caching it and using the cached value efficiently is somewhat
difficult, and not worth the complexity when the benefits would be
seen only on ancient kernels which have numerous other limitations and
bugs anyway.

make pthread attribute types structs, even when they just have one field

this change is to get the right tags for C++ ABI matching. it should
have no other effects.

fix some restrict-qualifier mismatches in newly added interfaces

these should have little/no practical impact but they're needed for
strict conformance.

add stub versions of some missing optional pthread interfaces

priority inheritance is not yet supported, and priority protection
probably will not be supported ever unless there's serious demand for
it (it's a fairly heavy-weight feature).

per-thread cpu clocks would be nice to have, but to my knowledge linux
is still not capable of supporting them. glibc fakes them by using the
_process_ cpu-time clock and subtracting the thread creation time,
which gives seriously incorrect semantics (worse than not supporting
the feature at all), so until there's a way to do it right, it will
remain as a stub that always fails.

debloat src/thread tree but putting lots of junk in one file

POSIX includes mostly-useless attribute-get functions for each
attribute-set function, presumably out of some object-oriented
dogmatism. the get functions are not useful with the simple idiomatic
usage of attributes. there are of course possible valid uses of them
(like writing wrappers for pthread init functions that perform special
actions on the presence of certain attributes), but considering how
tiny these functions are anyway, little is lost by putting them all in
one file, and some build-time cost and archive-file-size benefits are
achieved.