before SetEvent to eliminate a race.
(cygthread::terminate): Accumulate list of threads to check for termination and
call WaitForMultipleObjects on list rather than waiting for each thread
individually.
* sigproc.cc (subproc_init): Zap hwait_subproc thread handle since it is no
longer used.
* spawn.cc (spawn_guts): Fix so that cygthread::terminate is *really* called
only for exec.