df: ensure -a shows all remote file system entries

df: ensure -a shows all remote file system entries

commit v8.22-125-g9d736f8 printed placeholder "-" values
for device names that didn't match the preferred device name
for a particular mount point.  However that was seen to erroneously
suppress values for aliased host names or exports, common with
remote file systems.

* src/df.c (me_for_dev): Rename from devname_for_dev() so that
we can determine the remoteness as well as the name for the
preferred mount entry.
(get_dev): Don't output place holder values when both
current and preferred mount entries are remote.

Reported in http://bugs.debian.org/737399