dir: allow a path as is_mountpoint value

dir: allow a path as is_mountpoint value

This turns is_mountpoint more into export(5)'s `mountpoint`
property.
Given the directory storage with the properties:

    path /a/b/c
    is_mountpoint $value

$value = yes
    Same as before, /a/b/c must be mounted.
$value = no (or not set)
    Same as before, no effect.
$value = /a/b
    New: /a/b must be mounted (as opposed to /a/b/c)