findmnt: make udev usage more robust

 $ findmnt -rn --target /home/kzak/.bashrc -o UUID

uses libudev to ask for UUID, unfortunately /home/kzak is DM device
and libudev expects sysfs-like names (e.g. dm-0). We have to convert
the name to kernel devname.

Signed-off-by: Karel Zak <kzak@redhat.com>
lib/canonical: use realpath() from libc

.. after many many years it seems that we can follow libc code rather
than duplicate realpath(3) implementation.

Signed-off-by: Karel Zak <kzak@redhat.com>