Add feature: fileserver 3rd column in mapping.php

Add feature to read the fileserver per drive-letter from mapping.php. If
there is no third column  (i.e. fileserver) given for a drive-letter in
mapping.php, then the conf['fileserver'] value is used, if it is set.
This also ensures backward compatibility.

However. Since we can no longer be sure that there are only two values on
each valid line of mapping.php, we can no longer simply use confToHash,
but now have to manually reassemble two sets of lines: lines_fs and
lines_pathes.
Ensure mapping.php is empty for parser tests
Do not declare unused global $conf