* pthread.c: Use a third signal __pthread_sig_debug distinct
from __pthread_sig_cancel to notify gdb when a thread is
created
* manager.c: Likewise.
* internals.h: Likewise.
* signals.c: The implementation of sigwait(s) assumed that
all signals in s have signal handlers already attached.
This is not required by the standard, so make it work
also if some of the signals have no handlers.