ikiwiki hosting internals/ bugs/ external hostname validation

external hostnames need to be validated when changed -- we'll want to ensure that the DNS is properly set up to point to the site hostname. We'll probably want to at least warn about hardcoded IPs.

Validation will also help ensure that two sites don't try to generate conflicting apache config files, but by itself it's not sufficient. Consider:

- The user setups up a CNAME example.com pointing to site A.
- The user adds example.com to site A.
- The dns is changed to example.com now points to site B.
- The user adds example.com to site B.

Now ikisite will be generating conflicting apache config files. This scenario needs to be caught somehow, and doing so seems to call for a site-wide database.

On the ikisite side, I think a new subcommand is needed that handles setting external hostnames. It can do some of the validation. It is also needed to allow changing/adding an external hostname and adding/removing apache config files w/o having to 'ikisite disable' first, and 'ikisite enable' afterwards.

Update: `ikisite domains` sets up external hostnames/aliases, and does check the DNS. It also uses `ikisite sitelookup` to ensure that no other site is already configured to use the same external hostname.

While `ikisite sitelookup` still doesn't support multiserver use, I think what I did today is enough to call this bug done.