Fix orphaned processes when startup process fails during PM_STARTUP

Fix orphaned processes when startup process fails during PM_STARTUP

When the startup process exists with a FATAL error during PM_STARTUP,
the postmaster called ExitPostmaster() directly, assuming that no other
processes are running at this stage.  Since 7ff23c6d277d, this
assumption is not true, as the checkpointer, the background writer, the
IO workers and bgworkers kicking in early would be around.

This commit removes the startup-specific shortcut happening in
process_pm_child_exit() for a failing startup process during PM_STARTUP,
falling down to the existing exit() flow to signal all the started
children with SIGQUIT, so as we have no risk of creating orphaned
processes.

This required an extra change in HandleFatalError() for v18 and newer
versions, as an assertion could be triggered for PM_STARTUP.  It is now
incorrect.  In v17 and older versions, HandleChildCrash() needs to be
changed to handle PM_STARTUP so as children can be waited on.

While on it, fix a comment at the top of postmaster.c.  It was claiming
that the checkpointer and the background writer were started after
PM_RECOVERY.  That is not the case.

Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/CAJTYsWVoD3V9yhhqSae1_wqcnTdpFY-hDT7dPm5005ZFsL_bpA@mail.gmail.com
Backpatch-through: 15