* Fix a potential issue introduced with 415f4d3, namely that uv_spawn
  can fail when the current process is under job control. This would
  happen on Windows versions that don't support nested jobs (versions
  prior to Windows 8 / Server 2012).

* Change the `uv__init_global_job_handle` function signature to match
  what `uv_once` expects.

* Add a bunch of comments that clarify how we're using job control,
  and how we're dealing with job control that might be established by
  our parent process.

Use signaling mechanism for loop termination, because CFRunLoopStop() is
most likely not a thread-safe function and invoking it from other thread
may sometimes result in a "dead-lock".

fix #799

UV_EALREADY itself is already a libuv error, it should be set with
uv__set_artifical_error and not with uv__set_sys_error.

Closes #802

Conflicts:
	src/unix/darwin.c