Warn when using the error-prone EV_SIGNAL interface in an error-prone way. Also...

Warn when using the error-prone EV_SIGNAL interface in an error-prone way.  Also, fix a couple of race conditions in signal.c

When using the signal.c signal backend, Libevent currently only allows
one event_base to actually receive signals at a time.  (This has been
the behavior since at least 1.4 and probably much earlier.)  Now, we
detect and warn if you're likely to be racing about which signal goes
to which thread.

We also add a lock to control modifications of the evsig_base field,
to avoid race conditions like those found by Jason Toffaletti.

Also, more comments.  Comments are good.