A more-conservative workaround for Cygwin SIGCHLD issues.

A more-conservative workaround for Cygwin SIGCHLD issues.

* callproc.c (Fcall_process):
* process.c (create_process):
Make sure SIGCHLD is caught before we fork,
since Emacs startup no arranges to catch SIGCHLD.
* process.c (lib_child_handler): Initialize to null, not to
dummy_handler.
(catch_child_signal): Allow self to be called lazily.
Do nothing if it's already been called.
Assume caller has blocked SIGCHLD (all callers do now).
* emacs.c (main): Do not catch SIGCHLD here; defer it until
just before it's really needed.
* nsterm.m (ns_term_init): No need to re-catch SIGCHLD here,
since it hasn't been caught yet.