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).

Approved by:	re@ (rodrigc)
MFC r271577:

Fix descriptors leak.

PR:		bin/191002
Reported by:	Ryan Steinmetz
Submitted by:	mjg
Approved by:	re (gjb)