accept multiple --config requests for multiple channel groups

accept multiple --config requests for multiple channel groups

The previous UI implementation supported the selection of a channel
group, and defaulted to the device's global options. This let users
either manipulate global or a single channel group's parmeters.

Extend the user interface. Accept multiple -c specs. Apply the remaining
key-value pairs to the channel group which optionally can get specified
in the first item of a -c spec. This lets users modify several channel
groups' parameters in a single program invocation, including arbitrary
combinations including the device's global parameters.

This extension is completely backwards compatible. When a -c spec lacks
a 'channel_group=' item, then the -g spec is used, which defaults to the
global parameters.

Either an empty string or the 'global' literal can be used to select the
global parameters. Several -c specs can select the same channel group.
This can be useful when command lines get generated by other programs.