mtree: Make reading additional information from the fs optional

This feature is not needed if users just want to read in the content of
an mtree file and do validation against the file system themselves.

It is needed for `bsdtar cvf out.tar @input.mtree` which is why the
option is enabled in bsdtar.

Since the mtree tests rely on this feature, this patch also enables it
there.

Signed-off-by: Florian Pritz <bluewind@xinu.at>