Refactor: Unify config options for apc_modbus and other Modbus drivers (at least)

[jimklimov]

During work on #2294 I've noticed that apc_modbus, generic_modbus.c and adelsystem_cbi.c share several variables with different names for same concepts, notably for Serial-port RTU.

It may be wise to de-duplicate these (and also check if other drivers with serial port capability have their own names). Updated drivers should support old names for backwards-compatibility (and maybe warn that they are obsoleted - maybe a new variant of addvar() can be useful here) but not expose in help/docs.

Note also the documentation aspect in e.g. #2422 - that many options are specifically 10-based numbers, while others are guess-based (can be prefixed with 0x for base-16), see the last option in relevant strtol() parsers involved.

[jimklimov]

See also #2748