Never 4k align the MBR bootpool because zfsldr can not deal with a gap

If the bootpool does not start at the first sector of the BSD partition
then zfsldr seeks to the wrong offset inside the ZFS vdev label, and is
unable to find zfsboot, so the system does not boot

If 4k alignment is requested, align the BSD partition in the MBR table,
and align the swap and data pool, but the bootpool must start at sector 1

While here, if 4k alignment is requested, disable MBR CHS alignment, as
this results in not-4k aligned partitions.

Reported by: Alex Wilkinson
MFC after: 5 days
Sponsored by: ScaleEngine Inc.