Do not tickle glib SIGCHLD handling if Cygwin.

Do not tickle glib SIGCHLD handling if Cygwin.

This mostly consists of undoing recent changes.
* callproc.c (Fcall_process):
* process.c (create_process):
Do not worry about catching SIGCHLD here, undoing previous change.
* nsterm.m (ns_term_init): Re-catch SIGCHLD, undoing previous change.
* process.c, process.h (catch_child_signal):
No longer extern if !NS_IMPL_GNUSTEP, undoing 06-22 change.
* process.c (catch_child_handler): Don't worry about being called
lazily and do not assume caller has blocked SIGCHLD, undoing
previous change.  Move first-time stuff back to
init_process_emacs, undoing 06-22 change.  If CYGWIN, do not
tickle glib, as that causes Cygwin bootstrap to fail.  Do not
set lib_child_handler if it's already initialized, which may
help avoid problems on GNUStep.