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

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

Convert {small prefix}BSD to TERM=xterm as well.

Clean up the ttys files shipped with PicoBSD, NanoBSD and TinyBSD. While
there, it seems one of them still had references to sio(4). Make it in
sync with what we do in the base system.

Add files to build i386/amd64 bootable rescue ISO images using the
nanobsd framework.