Support gathering arbitrary display manager logs

Add support for gathering gdm and other display manager logs if the display manager is being currently used.

In xdiagnose 2.3 and earlier we collected gdm logs (as well as lightdm), but going forward we only collect the lightdm logs. Users who have upgraded from past releases will have the old gdm logs hanging around, but if the user is on lightdm those will be of no value.

The reason the gdm logs are no longer collected are threefold: 1. avoid collecting cruft files, 2. the logs are root-only readable due to potential for containing sensitive info (hostnames and so on) so dropping them slightly reduces risk of sensitive info leakage, and 3. the files are optional to begin with (we prompt the user if they want to include them). There is a very narrow, rare class of issue where we need these files (basically, if X is plain not starting), so having to manually ask for the files isn't going to burden us too much.

However, it would be nice to have a more generalized but discriminating mechanism to identify the correct display manager log files, in case the user is running xdm, gdm, or other display managers. It should also check timestamps on files and not include ancient ones.