s4-process_standard: Remove signal(SIGCHLD, SIG_IGN)

s4-process_standard: Remove signal(SIGCHLD, SIG_IGN)

We replace this with a pipe between parent and child, and then watch
for a read event in the parent to indicate that the child has gone away.

The removal of signal(SIGCHLD, SIG_IGN) requires us to then call
waitpid().  We can't do that in a main loop as we want to get the exit
status to the legitimate waitpid calls in routines like
samba_runcmd_*().

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>