geom_vfs: disallow multiple readonly mounts of a device

[Matthew Jacob]

>> Because of this, I propose to disable shared mounting feature for time being.
>> In my opinion it is very unlikely that anybody depends on this feature now, but
>> nullfs can be used as its replacement.  In fact, it could be even more efficient
>> if the same files are access via different mount points.
>>      

For me using nulls is the key point. This allows you to affect the name 
space for various policy reasons.