Add LINKS command

[progval]

This will be used by irctest to wait for sable_history to be up and reachable.

Other servers print a "description" from the configuration. But I'm guessing you aren't interested in adding that to the config, so I made it print some other info instead.

[spb]

I don't think this is the right way to approach the policy side. Long term, LINKS is an oper/management command, not a debugging one, so it belongs in that policy service. Long term the policy should also be configurable at runtime so that irctest can just specify an 'allow everything' policy for debug purposes, but that's quite a lot more work. I think for now the more sensible stepping stone is to put it in the oper policy service but always allow it, with a big 'TODO' label to address that down the road.

[progval]

I don't see it as an oper command because every other IRCd allows it for non-opers

[spb]

...and actually it's not a big deal either way if we're going to re-do the policy framework later anyway.