MFC r249566,r249644: EINTR in POSIX sem_*.

Document that sem_wait() can fail with [EINTR].

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 old POSIX semaphore implementation did this correctly, unlike the new
umtx one.

Specific to 9-stable: UMTX_ABSTIME does not exist and therefore
sem_timedwait() is erroneously not restarted after a SA_RESTART signal
handler.

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, as far as possible with
the ABI in 9-stable.
MFC r249979: intro(2): Fix some errors in ENFILE and EMFILE descriptions.
MFC r249956: sysconf(3): Correct the description of _SC_OPEN_MAX.
MFC r249859: getdtablesize(2): Describe what this function actually does.

getdtablesize() returns the limit on new file descriptors; this says nothing
about existing descriptors.
dup(2): Remove incorrect sentence about getdtablesize().

There are no getdtablesize() bounds on the file descriptor to be duplicated;
it only has to be open. If the RLIMIT_NOFILE rlimit was decreased after
opening the file descriptor, it may be greater than or equal to
getdtablesize() but still valid.