Rename ReservedBackends variable to SuperuserReservedConnections.

This is in preparation for adding a new reserved_connections GUC,
but aligning the GUC name with the variable name is also a good
idea on general principle.

Patch by Nathan Bossart. Reviewed by Tushar Ahuja and by me.

Discussion: http://postgr.es/m/20230119194601.GA4105788@nathanxps13

src/backend/utils/init/postinit.c

@@ -927,8 +927,8 @@ InitPostgres(const char *in_dbname, Oid dboid,
 	 * limited by max_connections or superuser_reserved_connections.
 	 */
 	if (!am_superuser && !am_walsender &&
-		ReservedBackends > 0 &&
-		!HaveNFreeProcs(ReservedBackends))
+		SuperuserReservedConnections > 0 &&
+		!HaveNFreeProcs(SuperuserReservedConnections))
 		ereport(FATAL,
 				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 				 errmsg("remaining connection slots are reserved for superusers")));