Sync with gnulib 3ea43e02541ece750ffc6cd1dfe34195421b4ef3.

mountlist: use /proc/self/mountinfo when available

Use libmount to propagate device IDs provided by Linux in
/proc/self/mountinfo.  This will give more accurate output when using df
in chroot'ed environments as the device IDs are not determined by stat()
which may be inaccurate within the chroot.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>