Guarantee that user signal callback is dispatched _after_ receiving a
signal as long as the handler expresses continued interest in receiving
such a notification.

Previously if a signal has been received during user callback dispatch
but before pending flag had been cleared then the signal would be
irrevocably lost.

This is a very useful guarantee to have in cases where signals are used
to signify a need for synchronization with external resources. For
example: reloading configuration file after SIGUSR1 or retrieving a
terminal size after SIGWINCH.