preforker: prevent call to 'write' on an fd that was already closed, plus minor code...

preforker: prevent call to 'write' on an fd that was already closed, plus minor code fixups

upon ceph_mon parent exiting it calls prefork.exit(err) that was resulting in
writing the exit status to fd[1] whis was previously closed in this process.
for fixing it, I guarded the call to 'signal_exit(r)' with 'if (is_child())'

on the way, I provided 2 minor code fixups in this file

Signed-off-by: Avner BenHanoch <avnerb@mellanox.com>