dmm/bm85x: introduce DMM packet parser for Brymen BM850(a/s)

dmm/bm85x: introduce DMM packet parser for Brymen BM850(a/s)

Implement a DMM packet parser for the BM850s protocol. This involves
variable length responses, which recently became a common serial-dmm
feature. Register the new parser under the "brymen-bm85x" device name.
This obsoletes the brymen-dmm driver which announces as "brymen-bm857".
This implementation was tested with the BM859s meter.

The text to number conversion with precision detection resolves the
last remaining issue of bug #1611.