Make setting type registration less icky; instead of having the
connection register all the settings, have the settings themselves
register that information at library load time. Putting this sort
of thing in G_DEFINE_TYPE_WITH_CODE is apparently more standard
than the home-rolled stuff we had before. Also document the
priority stuff so when adding new settings, people know what
priority to use.
(cleanups by jklimes)
Not all connections will require every secret, and sometimes we
can't automatically figure out whether we need the secret. For
vpnc sometimes the group password isn't used, and sometimes PPP
providers require a username but don't want a password, etc.
