protocol/ngircd: Support setting vhosts

Note that opers can set/clear +x themselves, and currently if Atheme
sets a vhost, then unsets it, and the oper sets +x manually, then
they'll end up with a blank hostname. This, however, should be fixed in
ngIRCd anyway.
Merge pull request #261 from grawity/ngircd-vhosts

protocol/ngircd: Support setting vhosts