df: ignore non file system entries in /proc/mounts

df: ignore non file system entries in /proc/mounts

Linux with network namespaces contains entries in /proc/mounts like:
  proc net:[4026532464] proc rw,nosuid,nodev,noexec,relatime 0 0
resulting in a failure to stat 'net:[...]', inducing a warning
and an exit with failure status.

* src/df.c (get_dev): Ignore all relative mount points.
* tests/df/skip-duplicates.sh: Add an entry to test relative dirs.