de8eff4d31
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)