Place the arguments at a fixed offset of 0x800 withing the argument area
(of size 0x1000). Allow variable size extended arguments first of which
should be a size of the extended arguments (including the size
parameter).
Consolidate all related definitions in a new i386/common/bootargs.h header.
Many thanks to jhb and bde for their guidance and reviews.
Reviewed by: jhb, bde
Approved by: jhb
MFC after: 1 month
