Make default signal backend fully threadsafe

Jason Toffaletti discovered with helgrind that our signal handler was
messing with evsig_base, which can be set from lots of places in the
code.  Ordinarly, we'd just stick a lock on it, except that it is
illegal (and genuinely error-prone) to call pthread_mutex_acquire()
from inside a signal handler.

The solution is to only store the fd we write to in a static variable,
write the signal number to the fd, and put evsig_cb in charge of
activating signal events.

I have no idea how we'll cope if we want to enable this to handle
siginfo (where available) in the future.

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