Add the ability to specify absolute and relative offsets to size partitions.

To create hybrid boot media we want to specify a partition at a known location.
This extends the syntax of size partitions to include an optional offset that
can be absolute or relative. It also introduces validation to make sure that
this hasn't resulted in overlapping partitions. I haven't added this to the
file and process partition specifications yet but the mechanics are designed
such that if someone comes up with a good way of specifying the offset it
will be fairly easy to add in.

Reviewed by: imp
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14916