fix #1757

Allow SAJOIN or implicit always-on join to override channels.operator-only-creation

irc/channelmanager.go

 		if entry == nil {
 			registered := cm.registeredChannels.Has(casefoldedName)
 			// enforce OpOnlyCreation
-			if !registered && server.Config().Channels.OpOnlyCreation && !client.HasRoleCapabs("chanreg") {
+			if !registered && server.Config().Channels.OpOnlyCreation &&
+				!(isSajoin || client.HasRoleCapabs("chanreg")) {
 				return nil, errInsufficientPrivs
 			}
 			// enforce confusables