fix(linux): Fix launch of km-config

km-config provides a .desktop file that allows the user to start
the configuration app like any other app. Unfortunately, a recent
change that improved the consistency of the manual pages broke
that because the .desktop file used to call km-config with the
parameter `-i` (plus file if given). With the recent change `-i`
now requires a filename, which caused `km-config` to output an error
and quit. For the user this looked like it didn't do anything.

This change adds the file/url as a new optional positional parameter
while keeping the existing `-i`/`--install` parameter.