Initialize child signal handling before posix_spawn too.

Initialize child signal handling before posix_spawn too.

Problem reported by Tino Calancha (Bug#58960).
* src/callproc.c (call_process): Initialize SIGCHLD handling
before possibly creating a child with emacs_span.  This need not
be in the critical section that calls emacs_spawn, so do it
outside the critical section.
* src/process.c (child_signal_init): Now extern.