sem: Restart the POSIX sem_* calls after signals with SA_RESTART set.

Programs often do not expect an [EINTR] return from sem_wait() and POSIX
only allows it if the signal was installed without SA_RESTART. The timeout
in sem_timedwait() is absolute so it can be restarted normally.

The umtx call can be invoked with a relative timeout and in that case
[ERESTART] must be changed to [EINTR]. However, libc does not do this.

The old POSIX semaphore implementation did this correctly (before r249566),
unlike the new umtx one.

It may be desirable to avoid [EINTR] completely, which matches the pthread
functions and is explicitly permitted by POSIX. However, the kernel must
return [EINTR] at least for signals with SA_RESTART clear, otherwise pthread
cancellation will not abort a semaphore wait. In this commit, only restore
the 8.x behaviour which is also permitted by POSIX.

Discussed with:	jhb
MFC after:	1 week