Sync with gnulub
c6148bca89e9465fd6ba3a10d273ec4cb58c2dbe.
mountlist: add me_mntroot field on Linux machines.
Populate me_mntroot in mount_entry so Linux machines based on
/proc/self/mountinfo can distinguish between bind mounts and original
mounts. In reality bind mounts aren't treated differently than
mountroot=/ mounts by the kernel, but the user often wants these bind
mounts distinguished.
Currently, this in not used in mc, but probably can be used in future.
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>