Make installboot on landisk aware of a possible MBR on the disk, and in this

Make installboot on landisk aware of a possible MBR on the disk, and in this
case install the first level bootstrap at the beginning of the of the wd0a
filesystem, rather than at the beginning of the disk.

Both locations work but the previous behaviour overwriting an existing MBR
is a violation of POLA.

tweaks & ok krw@