PTR records ending in . (e.g., `google-public-dns-b.google.com.`)
were considered invalid

I don't think casefolding things like `:6697` ever made sense.
Since these are configured by the ircd operator, it makes sense to
assume they'll already be in a canonical form regardless.

This matches the behavior of inspircd at the very least.

Previously, the comment above that section claimed ':' should be
disallowed, but the code didn't do so.

I also simplified the code a little bit and added tests.

Close the client's connection if we're unable to parse their IP.

This also simplifies the check to reduce indentation by a level.

Finally, this replaces the two-var construction of the pseudo-const
messages with an inline dereference via a slice to allow constructing
them less noisily.

This is a gotcha called out in the `time.After` docs. `time.After` will
leak the underlying channel if nothing ever receives on it.