mount: Turn into --external

Make --external support --ext-mount-map. The syntax is

 --ext-mount-map KEY:VAL == --external mnt[KEY]:VAL

Old option is kept for backward compatibility.

travis-ci: success for mnt: Deprecate --ext-mount-map for --external
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>