geom_label: Partially reinstate old sysinstall(8) workaround

This partially reverts commit af433832f7520840c22edd1fe1266c1a5cb781ad.
Since such bogus disklabels still exist in the wild, we now probe for a
disklabel to decide whether to ignore the UFS partition or not; if there
is a label then we use the old behaviour, and if there isn't one then we
use the new behaviour.

Reviewed by: cy, mckusick
Differential Revision: https://reviews.freebsd.org/D31068