Improve systemlyxdir handling

Improve systemlyxdir handling

Do not assume that the /systemlyxdir path prefix in \origin refers
to the system directory of the running instance, but check through
some heuristics what the real system dir is. In this way, a document
in the system dir of any other LyX installation is correctly spotted
and the \origin tag properly updated. For example, one can use an
installed version of lyx to edit a document in the lib/doc dir of a
git repo and obtain the same result as when running lyx in place.