This change forces `df(1)` to output each file system on a single
line by setting the `-P` flag (POSIX output format). Without it,
some versions of `df(1)` split the output into multiple lines if the
length of some fields like "Filesystem" exceeds a certain limit. As a
consequence, the `awk(1)` expression might fail to catch the correct
fields.
Please note that this fixes a file which is also wrongly named, which is
being addressed in PR #151.
Relates to PR #151.
Fixes issue #137.
