if only one session type is available, lightdm won't let you change preference for no-longer-present session type

[Steve Langasek]

If there is only one login session type available on the system, lightdm doesn't present you with the gear to configure your choice; it just logs you in to whatever is currently configured in .dmrc. This is a problem if the reason you only have one login session type is if you've *removed* one, and the .dmrc for the user you're trying to log in as points to a no-longer-existing session type.

If only one session type is present and the user's .dmrc points to a different session type, the existent session type should be used instead.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: lightdm 1.4.0-0ubuntu4
ProcVersionSignature: Ubuntu 3.8.0-6.13-generic 3.8.0-rc7
Uname: Linux 3.8.0-6-generic x86_64
ApportVersion: 2.8-0ubuntu4
Architecture: amd64
Date: Fri Feb 22 15:00:57 2013
InstallationDate: Installed on 2010-09-24 (882 days ago)
InstallationMedia: Ubuntu 10.04.1 LTS "Lucid Lynx" - Release amd64 (20100816.1)
MarkForUpload: True
SourcePackage: lightdm
UpgradeStatus: Upgraded to quantal on 2013-01-25 (28 days ago)
modified.conffile..etc.lightdm.lightdm.conf: [modified]
mtime.conffile..etc.lightdm.lightdm.conf: 2013-02-02T01:19:48.995495