Fix checksum calculation:
1.  Iterate over all partitions counted in the label, which can be more
    than the number of partitions given to mkimg(1).
2.  Start the checksum from the beginning of the label; not the beginning
    of the bootarea.

Tested with bsdlabel(8).

MFC after:	3 days
Be compatible with boot code that starts right after the disk label in
the second sector by only clearing the amount of bytes needed for the
disklabel in the second sector. Previously we were clearing exactly 1
sector worth of bytes and as such writing over boot code that may have
been there.
Since we do support more than 8 partitions, make sure to set all fields
in d_partitions. For the first 8 partitions this is unneeded, but for
partitioons 9 and up this compensates for the fact that we don't clear
an entire sector anymore.
Obviously, one cannot use more than 8 partitions when using boot code
that starts right after the disk label.

Relevant GRNs:
107879 - Employ unused bytes after the disklabel in the second sector.
189500 - Revert the part of change 107879 that employs the unused bytes
	 after the disklabel in the 2nd sector for boot code.

Obtained from:	Juniper Networks, Inc.
MFC after:	3 days