cmdline: add listconfig/helpconfig

[ecdsa]

This adds commands to view the list of config variables

[accumulator]

These commands should not require a running daemon, but currently require either a running daemon or an explicit --offline (but I guess this applies to more commands, as e.g. setconfig/getconfig also require --offline currently)

[accumulator]

minor nitpick: helpconfig use_gossip returns Use trampoline routing

[ecdsa]

The --offline syntax requirement is off-topic. This syntax is explicitly needed for all commands, regardless of whether they use the network, because it clarifies whether the command uses the running daemon or not.