Tidy up waiting for clients on shut down

Signal client threads to wake up out of any pending poll(), and wait for
them all to go away on shutdown.
perform stronger validation on ignore_vcs setting during watch

Rather than passively moan about a problem to the log file,
cause a watch attempt to fail if the ignore_vcs setting is bad.
fix non-DESTDIR install case

Addresses https://github.com/facebook/watchman/issues/39