Try to avoid malloc SEGVs on Cygwin.

Try to avoid malloc SEGVs on Cygwin.

* callproc.c, process.h (block_child_signal, unblock_child_signal):
Now extern.
* emacs.c (main): Catch SIGCHLD just before initializing gfilenotify.
* process.c (catch_child_signal): Block SIGCHLD while futzing with
the SIGCHLD handler, since the code is not atomic and (due to glib)
signals may be arriving now.
* sysdep.c (init_signals): Do not catch child signals here;
'main' now does that later, at a safer time.