geom: Allow BSD type '!0' partitions

[jlduran]

Summary

Allow the creation of partition type '!0', in order to replicate the
behavior of bsdlabel -w.

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

Test Plan

md0=$(mdconfig -s 100m)
md1=$(mdconfig -s 100m)

bsdlabel -w "$md0"

gpart create -s bsd "$md1"
gpart add -t '!0' -b 16 "$md1"

gpart show
=>     0  204800  md0  BSD  (100M)
       0      16       - free -  (8.0K)
      16  204784    1  !0  (100M)

=>     0  204800  md1  BSD  (100M)
       0      16       - free -  (8.0K)
      16  204784    1  !0  (100M)

[emaste]

I think this is reasonable -- 0 is unused/reserved, but no reason we shouldn't be able to specify it. It's not out of range etc.