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

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

MFp4: Integrate a boatload of bug fixes from p4. We're right on the
8k boundary with this program still.

text data bss dec hex filename
7925 4 4476 12405 3075 bootiic.out

so we have like 293 bytes left before we have to play games. There
may be ways to reduce that somewhat, but they start to be very board
specific.