Change pthread::cancelable_wait to just cancelable_wait, throughout.

Change pthread::cancelable_wait to just cancelable_wait, throughout.
* thread.h (cw_sig_wait): New enum.
(fast_mutex::lock): Use cancelable_wait with resumable signal.
(cancelable_wait): Change fourth argument to cw_sig_wait enum.
* thread.cc (cancelable_wait): Ditto.  Loop on signal detection if fourth
argument == cw_sig_resume.