Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

btxldr: future-proof argument passing from boot1/2-ish to loader

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

Move boot2 BSS zeroing into btx startup code out of boot1. boot1 does not
have clear idea on boot2 BSS size and leaves portion of it not zeroed out.
btxcsu.s is in much better position for this job.

Obtained from: DragonflyBSD (with minor adjustments)

$Id$ -> $FreeBSD$

Adjust setting of argument pointer for BTX 0.87.

Add BTX startup/interface code.