Use the mountpoint command when available
Use the mountpoint command when available

Our Go implementation doesn't appear to catch every possible corner
cases, in my case, a bind-mount from the same underlying tmpfs as the
parent directory.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

lxc/lxd-pkg-ubuntu,lxc/lxd-pkg-ubuntu,lxc/lxd-pkg-ubuntu