* exceptions.cc (call_handler): Use new muto linked list to look for all

* exceptions.cc (call_handler): Use new muto linked list to look for all
potential mutos owned by suspended thread.  Clear waiting threads while thread
is stopped.
(proc_subproc): Clarify debugging output.
* sync.h (class muto): Add 'next' field.
(new_muto): Keep linked list alive.