In the cleanup-after-error section of uv_spawn(), check that the pointer
is non-NULL - we might end up in said section due to a malloc() failure.

The cleanup-after-error code path in uv_spawn() was closing file
descriptors indiscriminately.  Only close file descriptors that we
created ourselves, not the ones that are passed in by the user.

Fixes nodejs/node-v0.x-archive#6297.

This commit reverts the following commits:

    983fa68 darwin: fix 10.6 build error in fsevents.c
    684e212 fsevents: use shared FSEventStream
    ea4cb77 fsevents: FSEvents is most likely not thread-safe
    9bae606 darwin: create fsevents thread on demand

Several people have reported stability issues on OS X 10.8 and bus
errors on the 10.9 developer preview.

See also nodejs/node-v0.x-archive#6296 and nodejs/node-v0.x-archive#6251.

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.

Conflicts:
	include/uv-darwin.h
	src/unix/fsevents.c
	src/unix/process.c