Clean up SIGCHLD handling a bit.

Clean up SIGCHLD handling a bit.

* process.c, process.h (catch_child_signal):
Now always extern, even if !NS_IMPL_GNUSTEP.
* process.c (catch_child_signal): Move glib tickler here from
init_process_emacs, so that it's done earlier in Emacs
initialization.  Also move the noninteractive && !initialized
check here from init_process_emacs.  This is all a bit cleaner for
GNUish platforms, and I hope it works around the Cygwin bug.
* sysdep.c (init_signals): Invoke catch_child_signal here, so
that glib signal handling is tickled before glib creates threads.