This is a proposed change to uber-go#912 by @jasonmills
that DRYs up internal state management by unifying
`chan os.Signal` and `chan ShutdownSignal` into a single interface
as suggested in this comment:
uber-go#912 (comment)

This change isn't quite right because mapping os.Signal to a
ShutdownSignal currently relies on a goroutine
which isn't reliably shut down -- so we have leaking tests.

Note that this also fixes a behavioral bug in uber-go#912:
`Wait()` channels would not resolve if a plain signal was received.

…y using mutex