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

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

Use the new command file feature of ddb(8) to support setting ddb(4)
scripts at boot. This is currently disabled by default. /etc/ddb.conf
contains some potentially reasonable default scripts.

PR: conf/119995
Submitted by: Scot Hetzel <swhetzel at gmail dot com> (Earlier version)
X-MFC after: textdumps