Optimise handling of select() returning <= 0.

Optimise handling of select() returning <= 0.

Before, we immediately retried select() if it returned -1 and errno is EAGAIN
or EINTR, and if it returned 0 it would check for network events even if we
know there are none.  Now, if -1 or 0 is returned we skip checking network
events, but we do check for timer and signal events.