Start the default OpenBSD partition either immediately following
any boot partition specified via '-b' or /usr/mdec/mbr; at the
power of 2 block after the first track; or immediately following
the MBR if there is only one track.
Mark any non-EFISYS boot partition created by -b as DOSACTIVE.
Suggested by kettenis@, better than a separate new option.
Brings -b behaviour into line with many uses of -e to create boot
partitions, allowing for the eventual elimination of said -e uses
in the creation of the various boot media and in the install
scripts.