s3:smbd: let smbd/nmbd/winbindd child processes terminate if the parent process died.

s3:smbd: let smbd/nmbd/winbindd child processes terminate if the parent process died.

This applies to all child processes making use of reinit_after_fork().
It is implemented by establishing a pipe between parent and child.
The child watches for EOF on the read end of the pipe, indidcating
an exited parent.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>