Refactor modbus drivers: use NUT common code for ModBus over USB link

[jimklimov]

In spirit of #1763 consider providing driver configuration options to help match the devices.

Note that currently the point may be moot, with several independent drivers (phoenixcontact_modbus generic_modbus huawei-ups2000 socomec_jbus adelsystem_cbi) not even mentioning "usb" in their sources, so it is possibly offloaded to libmodbus build installed into the end-user OS. This may however be a consideration of we proceed with #50 to unify them as subdrivers under a single binary.