df: dereference symlinks to disk device nodes

df: dereference symlinks to disk device nodes

This is so the matching for the device is done on the canonical name
of the disk node, rather than on the path of the symlink.
In any case the user will generally want to use the symlink target.

* src/df.c (get_disk): Canonicalize the passed file,
before matching against the list of mounted file system devices.
Note we pass the original symlink name to the "file" output field,
as the symlink target is usually available through the "source" field.
* tests/df/df-symlink.sh: Test the dereferencing operation.
* tests/local.mk: Mention the new test.
* NEWS: Mention the fix.
Reported by Ondrej Oprala