interfaces: performance for https://github.com/opnsense/core/issues/3567
interfaces: performance for https://github.com/opnsense/core/issues/3567

* interfaces_addresses() only request single interface data when there's only one interface requested
* interface_configure() mtu check only needs a specific interface, don't request all

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core