vinumdrive geom with an exclusive bit. This should fix the problem
when underlying partitions overlap (i.e. the 'a' partition is at
the same offset as the 'c' partition).
Ideas borrowed from pjd@, quite a bit of testing by
Matthias Schuendehuette <msch@snafu.de>.
allocated ressouces should be ultimately freed in gv_destroy_geom()
(when unloading the module and not earlier), but I need to look at this
more closely.
