This is a second try at what commit 57431a91 tried to do, namely,
launch the syslogger before we open postmaster sockets so that our
messages about the sockets end up in the syslogger files.  That
commit fell foul of a bunch of subtle issues caused by trying to
launch a postmaster child process before creating shared memory.
Rather than messing with that interaction, let's postpone opening
the sockets till after we launch the syslogger.

This would not have been terribly safe before commit 7de19fbc,
because we relied on socket opening to detect whether any competing
postmasters were using the same port number.  But now that we choose
IPC keys without regard to the port number, there's no interaction
to worry about.

Also delay creation of the external PID file (if requested) till after
the sockets are open, since external code could plausibly be relying
on that ordering of events.  And postpone most of the work of
RemovePgTempFiles() so that that potentially-slow processing still
happens after we make the external PID file.  We have to be a bit
careful about that last though: as noted in the discussion subsequent to
bug #15804, EXEC_BACKEND builds still have to clear the parameter-file
temp dir before launching the syslogger.

Patch by me; thanks to Michael Paquier for review/testing.

Discussion: https://postgr.es/m/15804-3721117bf40fb654@postgresql.org