Better error handling for login mgrs in server install/uninstall

Better error handling for login mgrs in server install/uninstall

The purpose is to catch it when either no modules are enabled or if
you try to set the login module order and one of them is not
available/installed, then fail gracefully.

There were some baked-in assumptions that all login providers
are installed. Add some error handling around trying to determine
what is available, and rather than trying to force pam to be enabled
just exit with a handy message.

Don't rely on lm_order during uninstall. Use the list of enabled
Login managers instead.

Bail out of argument checking if uninstall is requested.

https://fedorahosted.org/ipsilon/ticket/105

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>