geom: Allow BSD type '!0' partitions

Allow the creation of '!0' partition types.

Fix it by not considering "0" an invalid partition type.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47652

(cherry picked from commit accf71534c612b76ee2701f2dfcaa464748e527a)