postmaster: Start syslogger earlier

When the syslogger was originally
added (bdf8ef69), nothing was normally
logged before the point where it was started.  But since
f9dfa5c9, the creation of sockets
causes messages of level LOG to be written routinely, so those don't
go to the syslogger now.

To improve that, arrange the sequence in PostmasterMain() slightly so
that the syslogger is started early enough to capture those messages.

Discussion: https://www.postgresql.org/message-id/d5d50936-20b9-85f1-06bc-94a01c5040c1%402ndquadrant.com


Reviewed-by: Christoph Berg <christoph.berg@credativ.de>