irclib: Tweak message queue priorities

- Move PRIVMSG and NOTICE from low to normal priority to eliminate
  on-connect lag
- Add CAP to high priority to match CAPAB
Misc: handle renaming of "help" command
Misc: fix a typo (s/uncompatible/incompatible/)
Merge pull request #934 from GLolol/misc/fix-typo

Misc: fix a typo (s/uncompatible/incompatible/)
Merge branch 'misc/customhelp', remote-tracking branches 'upstream/testing' and 'nyuszika7h/tweak-queue-priorities' into lilybot