shutdown: use libmount to enumerate /proc/self/mountinfo

shutdown: use libmount to enumerate /proc/self/mountinfo

This is analogous to 8d3ae2bd4c9bf9fc2e57f7b3776325a1c750ca30, except that now
src/core/umount.c not src/core/mount.c is converted.

Might help with https://bugzilla.redhat.com/show_bug.cgi?id=1554943, or not.

In the patch, mnt_free_tablep and mnt_free_iterp are declared twice. It'd
be nicer to define them just once in mount-setup.h, but then libmount.h would
have to be included there. libmount.h seems to be buggy, and declares some
defines which break other headers, and working around this is more pain than
the two duplicate lines. So let's live with the duplication for now.

This fixes memleak of MountPoint in mount_points_list_get() on error, not that
it matters any.