systemctl: warn if trying to disable a unit with no install info

systemctl: warn if trying to disable a unit with no install info

Trying to disable a unit with no install info is mostly useless, so
adding a warning like we do for enable (with the new dbus method
'DisableUnitFilesWithFlagsAndInstallInfo()'). Note that it would
still find and remove symlinks to the unit in /etc, regardless of
whether it has install info or not, just like before. And if there are
actually files to remove, we suppress the warning.

Fixes #17689