Fix support for /msg @#chan :hi

005 STATUSMSG defines list of characters prependable to channel name,
but we used simple modes instead.

See
https://tools.ietf.org/html/draft-brocklesby-irc-isupport-03#section-3.16

Fix #272

Thanks to grawity for the link to 005 docs draft, and to carrot for
testing the patch.