Fix exactly <limits:maxchan> long channel names being truncated

Fixes #422 reported by @RawrDragon
cmd_invite Correct error message when the user has no privs to invite and no halfop prefix is available
cmd_invite Send invite announcements to halfops and higher when <security:announceinvites> is 'dynamic' regardless of the channel being +i or not

Clarify what the various options mean in the doc

Fixes #418 reported by @RawrDragon