Add ability to reload the channel_manager.

While we're at it, this may as well be reloadable too (particularly
because I'm planning on adding some new functionality).  This
requires a tiny bit of cleverness to stuff the existing channels
back into the new channel_manager, but that's easy enough.
Add new 'channels' command to list your channels.

This was missing before.  Still not super-elegant, but it does the
job.
Comment fix in reload_login().

Just because it was cut-and-pasted from reload_chat() doesn't mean
it shouldn't have proper comments.
Deduplicate admin_manager's admin() function.

It had a bunch of cut-and-paste code which, unlike the actual
function calls, really was just the exact same thing over and
over with a couple of string differences.  Easy to deduplicate.