Standardize nut_ser_addvars() for common code and documentation

[jimklimov]

We have a number of serial-port communications capable drivers, each of which reinvents the wheel about the set and names of parameters involved in the connection set-up (speed, parity, stop-bits, etc.)

There is a nice precedent with nut_usb_addvars() adopted by most USB-capable drivers (with a couple exceptions at the moment), which standardizes and simplifies both the code (drivers themselves, C++ parser bindings, augeas lenses...) and man page documentation inlays.

This issue is to track a similar idea for serial-port drivers (including multiple-capability programs like nutdrv_qx or *modbus drivers), deprecating obscure option names where needed (still accepting them as aliases though).