unix: revert recent FSEvent changes

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 joyent/node#6296 and joyent/node#6251.