ircv3: Add support for weird tag names, per the spec

IRCv3 currently does not specify (or recommend specifying) any tag
names which would contain multiple slashes, but IRCv3 does recommend
clients should gracefully handle any possible tag, ideally treating
it as opaque string.

We’d like to avoid that, but also want to ensure even after a roundtrip
through our core, tags we don’t support are still in identical
condition.