When this option is specified, if an external (private) bind mount is not
specified by --ext-mount-map KEY:VAL then it is attempted to be resolved
automatically.

v2: introduce find_best_external_match, which looks for the best match based on
    sharing/slave ids; don't try to resolve fsroot_mounted() mountpoints
v3: get rid of really_collect_self_mounts
v4: get rid of fsroot_mounted() check when autodetecting external mounts
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>