DNS: zone serial number management

Right now, serial number management is basically left to the user. There's an "increment_serial" function available in the backend, but it's not used anymore. The way this should work is that the backend should *parse* the zonefile and actually increment from *that* serial when editing a zonefile. That serial number would then be returned to the frontend, which would stored the modified serial in the database.

From a user perspective, the serial number should be a readonly field.