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.
* lib/mountlist.c (read_file_system_list): Use the libmount routines
from util-linux to parse "/proc/self/mountinfo" or fall back to
standard getmntent() processing.
* m4/ls-mntd-fs.m4: Check for libmount only when 1-argument
getmntent() is used, as is the case on GNU/Linux.
* DEPENDENCIES: Mention the optional util-linux dependency.