Cope with distro version "93u-28.2.2"

Also, just log a warning if try_cleanup_distro_version returns an invalid version.

Seen in OpenSUSE's ksh.
Write old-style format header for old-style cache files

On RPM systems, it would keep regenerating the RPM cache.
Fail with an error if --gui is used and the GUI plugin can't be loaded

Before, we just fell back to console mode in all cases.