[Patch] Create a new session in postmaster by calling setsid()

Hello,

Recently I encountered an issue during testing and rootcaused it as the
title mentioned.

postmaster children have done this (creating a new session) by calling
InitPostmasterChild(),
but postmaster itself does not. This could lead to some issues (e..g signal
handling). The test script below is a simple example.

$ cat test.sh
pg_ctl -D ./data -l stop.log stop
sleep 2 -- wait for pg down.
pg_ctl -D ./data -l start.log start
sleep 2 -- wait for pg up.
echo "pg_sleep()-ing"
psql -d postgres -c 'select pg_sleep(1000)'  -- press ctrl+c, then you will
see postmaster and its children are all gone.

Long ago PG has pmdaemonize() to daemonize postmaster which of course
create a new session. That code was removed in the patch below.

commit f7ea6beaf4ca02b8e6dc576255e35a5b86035cb9
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date:   Mon Jul 4 14:35:44 2011 +0300

    Remove silent_mode. You get the same functionality with "pg_ctl -l
    postmaster.log", or nohup.

    There was a small issue with LINUX_OOM_ADJ and silent_mode, namely that
with
    silent_mode the postmaster process incorrectly used the OOM settings
meant
    for backend processes. We certainly could've fixed that directly, but
since
    silent_mode was redundant anyway, we might as well just remove it.

Here is the related discussion about the patch. It seems that is related to
oom score setting in fork_process().
https://www.postgresql.org/message-id/4E046EA5.8070101%40enterprisedb.com

Personally I still like pg being a daemon, but probably I do not know some
real cases which do not like daemon. If we do not go back to pgdaemonize()
with further fix of fork_process() (assume I understand correctly) we could
simply call setsid() in postmaster code to fix the issue above. I added a
simple a patch as attached.

Thanks.