Restore fast-path event_reinit() for slower backends

Restore fast-path event_reinit() for slower backends

We used to use the needs_reinit flag in struct eventop to indicate
whether an event backend had shared state across a fork(), and
therefore would require us to construct a new event backend.  But
when we realized that the signal notification fds and the thread
notification fds would always be shared across forks, we stopped
looking at it.

This patch restores the old behavior so that poll, select, and
win32select don't need to do a linear scan over all pending
fds/signals when they do a reinit.  Their life is hard enough
already.