utillinux: fixing mount/umount, for our case of /etc/mtab symlinking to /proc/mounts

In systemd, without this patch, 'mount' for 'user' fstab devices works, but
umount does not; it says to require root.
All gets normal with this patch.

pkgs/os-specific/linux/util-linux/default.nix

@@ -21,10 +21,13 @@ stdenv.mkDerivation rec {
   # (/sbin/mount.*) through an environment variable, but that's
   # somewhat risky because we have to consider that mount can setuid
   # root...
+  # --enable-libmount-mount  fixes the behaviour being /etc/mtab a symlink to /proc/monunts
+  #     http://pl.digipedia.org/usenet/thread/19513/1924/
   configureFlags = ''
     --disable-use-tty-group
     --enable-write
     --enable-fs-paths-default=/var/setuid-wrappers:/var/run/current-system/sw/sbin:/sbin
+    --enable-libmount-mount
     ${if ncurses == null then "--without-ncurses" else ""}
   '';