(call_handler): Allow signals to be sent even if signalled thread is stopped.
Change order of signal_arrived arming/waiting threads clearing to eliminate a
race.
(reset_signal_arrived): New helper function.
* malloc.cc (malloc_init): Use mutos so that signal handler can keep track of
who owns the lock.
(__malloc_lock): Ditto.
(__malloc_unlock): Ditto.
* sync.h (new_muto): Actually use a muto for the "buffer".
* Makefile.in: Fix a dependency.
