Teach ufs_module.c about bsd labels and probe 'a' partition.

If the check for a UFS partition at offset 0 on the disk fails, check
to see if there's a BSD disklabel at block 1 (standard) or at offset
512 (install images assume 512 sector size). If found, probe for UFS
on the 'a' partition.

This fixes UEFI booting images from a BSD labeled MBR slice when the
'a' partiton isn't at offset 0. This is a stop-gap fix since we plan
on removing boot1.efi in FreeBSD 12. We can't easily do that for 11.2,
however, hence the short MFC window.

Tested by: emaste@
MFC After: 3 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D15598