Remove external usages of prefix mode bitmaps.

Prefix modes are now stored as a string, with the most important
mode at the start (e.g. "ohv").

Change-Id: Ie51e6bf8bf7a46e80295ed5cf000ffb4b0195e0b
Reviewed-on: http://gerrit.dmdirc.com/3954
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager