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.

update 'sio' with 'uart' and apply related changes.

Uncomment the entry for a getty on the serial port. Boxes with
a serial console can make use of it.

Add a (commented out) sample entry for use with a serial line.

Cleanup of the default floppy tree for PicoBSD builds.

Approved-by: jordan

Major PicoBSD update by luigi with some debugging help from myself.

. Now builds on -STABLE (-CURRENT is broken due to bugs)
. etc directory contents centralized instead of in each
type directory (can exclude & override as desired)
. Removed extraneous language files (lang files for rc really necessary?)
. dialog-based build tool with support for custom floppy builds
. MFS image loads as a mfs_root module instead of compiled into kernel

THIS IS BROKEN ON CURRENT. I'll MFC to -STABLE immediately following.
luigi tells me I have an OK from jkh on the MFC.

Submitted by: luigi