Clean up semaphore EINTR handling after Linux futex docs clarification.

Clean up semaphore EINTR handling after Linux futex docs clarification.

The Linux kernel futex documentation now states that since Linux 2.6.22,
FUTEX_WAIT does return EINTR only when interrupted by a signal, and not
spuriously anymore.  We only support more recent kernels, so clean up
EINTR handling in the semaphore and update the comments.