libratbox: Add comment that case fallthrough is deliberate.
bandb: Don't apply empty ban list when bandb starts sending bans.

When bandb sends the ban list, it first sends 'C', then all bans and
finally 'F'. Only when 'F' is sent is ircd supposed to apply the bans.
Because of a missing break, 'C' also did 'F', clearing the ircd active
permanent bans until bandb sent 'F'.

The effect is pretty limited because having bandb send the ban list via
/rehash bans is uncommon and most bans will be enforced when reset.