initdb authentication

Ok, here is one more try at the initdb default authentication stuff.
This one adds the switches "--ident" and "--trust", which will configure
pg_hba.conf with ident and trust authentication respectively. If trust
authentication is selected, a warning is written to pg_hba.conf. The old
switches for password prompt/file still apply.

This one makes it mandatory to pick some kind of authentication. If
that's not wanted, it's easy to change it to default to trust (which I
think is wrong, but we've been through that already..)

Oh, and this time, the comments are updated :-)

//Magnus