add __timedwait backend workaround for old kernels where futex EINTRs

add __timedwait backend workaround for old kernels where futex EINTRs

prior to linux 2.6.22, futex wait could fail with EINTR even for
non-interrupting (SA_RESTART) signals. this was no problem provided
the caller simply restarted the wait, but sem_[timed]wait is required
by POSIX to return when interrupted by a signal. commit
a113434cd68ce30642c4995b1caadcd084be6f09 introduced this behavior, and
commit c0ed5a201b2bdb6d1896064bec0020c9973db0a1 reverted it based on a
mistaken belief that it was not required. this belief stems from a bug
in the specification: the description requires the function to return
when interrupted, but the errors section marks EINTR as a "may fail"
condition rather than a "shall fail" one.

since there does seem to be significant value in the change made in
commit c0ed5a201b2bdb6d1896064bec0020c9973db0a1, making it so that
programs that call sem_wait without checking for EINTR don't silently
make forward progress without obtaining the semaphore or treat it as a
fatal error and abort, add a behind-the-scenes mechanism in the
__timedwait backend to suppress EINTR in programs that have never
installed interrupting signal handlers, and have sigaction track and
report this state. this way the semaphore code is not cluttered by
workarounds and can be updated (to be done in next commit) to reflect
the high-level logic for conforming behavior.

these changes are based loosely on a patch by Markus Wichmann, with
the main changes being atomic update to flag object and moving the
workaround from sem_timedwait to the __timedwait futex backend.