Fix mounting of external directories into /esg/gridftp_root/ when $mount...

..._dir is found in 'mount -l' output. Ensures the filtered pattern occurs at the beginnig of the 'mount -l' output. This allows for multiple mountings with the same $mount_dir, e.g. if /data is already mounted into the base system configuration and one will have it also have mounted into /esg/gridftp_root.

Tested on esg.pik-potsdam.de