Change infrastructure for users, groups and channels

Dropping surrogate keys for groups, channels and channel flags. Instead
use the following changes.

* groups.flag changes name to groups.gid and is used as primary key.
  Also attributes referencing this attribute with foreign keys are named
  gid.

* channels.name changes name to channels.channel and is used as primary
  key.

* channel_flags gets a flag attribute used as primary key.

Change username, pnick and hostaname to use the citext type, also use
normal unique constraints instead of functional indexes on lower().

Since ships_home has to be dropped and recreated, also change this view and
available_ships to use the new infrastructure.