unix, windows: add uv_fs_event_start/stop functions

Make uv_fs_event behave like other handles, that is, it's inactive after
init, and it's active between start and stop.
unix: fix non-synchronized access in signal.c

Check the return value of uv__signal_lock(); don't mutate the signal
watcher tree in the signal handler if we failed to acquire the lock.
Merge remote-tracking branch 'origin/v0.10'

Conflicts:
	include/uv-darwin.h
	src/unix/fsevents.c
	src/unix/process.c
Adds <stdlib.h> to satisfy reference to abort.
build: Fixes check target for mingw.
test: Removes replacement snprintf for mingw.

Mingw has a proper definition of snprintf.