Split nmci.ctx into multiple files

Regroup functions from nmci.ctx to existing and new nmci modules, where it fits the most. Names are open for discussion. Each change has its own commit, so it is easy to change/drop something.

There are still some functions in nmci.ctx, maybe testeth0 related functions can be move to nmutil (as it controls testeth0 connection via nmcli), or add also nmci.veth and it would suit there together with some other vethsetup related functions.

Then, there is also dump_status and update_udevadm. If we move veths out, maybe this 2 functions could be in nmci.util? It is open for discussion.