Switch init to epoll.

Not just because it's what the cool kids are doing --- it also lets us
simplify the inner loop and decouple it from whatever systems want to
be woken to perform some activity if there's data to be read on some fd.

Currently this is just used to clean up the existing signal handling,
keychord, and property service code.

Change-Id: I4d7541a2c4386957ad877df69e3be08b96a7dec5