Log PostgreSQL version number on startup

Logging the PostgreSQL version on startup is useful for two reasons:
There is a clear marker in the log file that a new postmaster is
beginning, and it's useful for tracking the server version across
startup while upgrading.

Author: Christoph Berg <christoph.berg@credativ.de>
Discussion: https://www.postgresql.org/message-id/flat/20181121144611.GJ15795@msg.credativ.de/

src/backend/postmaster/postmaster.c

@@ -1052,6 +1052,10 @@ PostmasterMain(int argc, char *argv[])
 
 	whereToSendOutput = DestNone;
 
+	/* Report server startup in log */
+	ereport(LOG,
+			(errmsg("starting %s", PG_VERSION_STR)));
+
 	/*
 	 * Establish input sockets.
 	 *