Tidied up the joining behaviour a bit.

Tidied up the joining behaviour a bit.

A few changes:

 * Burst with the default modes as well as the forcemodes (unless some of
   the default modes are denied, of course).

 * If there's only one user on the channel and they are known, don't burst
   with +i or +k - this avoids kicking the users off their own channels.
   Obviously if the user isn't known they shouldn't be joining so we leave
   the modes set to get rid of them

 * Ripped out the #ifdef 0'd code that used to try and deal with imposters -
   there is no need for this now that we can burst into channels.