decode: assign logic channels to PD inputs (by name or by index)

Add support for the -P <name>:assign_channels=auto_index or =auto_names
keywords. Which assigns sigrok logic channels to decoder input signals,
but requires explicit initiation by users. The default behaviour remains
backwards compatible, exclusively assigns individual channels as mapped
by user specs, assigns nothing by default.

Name matching is implemented as a simple case insensitive comparison.
Neither weaker name checks (optional separators) nor advanced heuristics
are supported as seen in the Pulseview GUI application. Future versions
could implement these once the set of transformation rules has settled.