extends util/config and util/pconfig

Created by: Treer

Gives util/pconfig the ability to list all the channel IDs, and adds command-line help to both utils.

Before merging this, someone who's more familiar with hubzilla should check my assumptions:

• these scripts are only intended to be used from the command-line, so improving their command-line UI should be safe with regard to the rest of hubzilla
• There'll never be a configuration family named "-h" or "--help", so util/config -h is unambiguous. (if "-h" isn't the first argument then getopt() will ignore it)