Refactor InitPostgres() to use bitwise option flags

InitPostgres() has been using a set of boolean arguments to control its
behavior, and a patch under discussion was aiming at expanding it with a
third one.  In preparation for expanding this area, this commit switches
all the current boolean arguments of this routine to a single bits32
argument instead.  Two values are currently supported for the flags:
- INIT_PG_LOAD_SESSION_LIBS to load [session|local]_preload_libraries at
startup.
- INIT_PG_OVERRIDE_ALLOW_CONNS to allow connection to a database even if
it has !datallowconn.  This is used by bgworkers.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZSTn66_BXRZCeaqS@paquier.xyz

src/backend/tcop/postgres.c

@@ -4206,11 +4206,12 @@ PostgresMain(const char *dbname, const char *username)
 	 * NOTE: if you are tempted to add code in this vicinity, consider putting
 	 * it inside InitPostgres() instead.  In particular, anything that
 	 * involves database access should be there, not here.
+	 *
+	 * Honor session_preload_libraries if not dealing with a WAL sender.
 	 */
 	InitPostgres(dbname, InvalidOid,	/* database to connect to */
 				 username, InvalidOid,	/* role to connect as */
-				 !am_walsender, /* honor session_preload_libraries? */
-				 false,			/* don't ignore datallowconn */
+				 (!am_walsender) ? INIT_PG_LOAD_SESSION_LIBS : 0,
 				 NULL);			/* no out_dbname */
 
 	/*