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

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

Comments.

Improve the debugging abilities and clean up debug messages. In most cases,
all one has to do is set the environment variable DEBUGGING to get the debug
messages to appear on the console.

Import bsdconfig(8) as a replacement for the post-install abilities of
deprecated sysinstall(8). NOTE: WITH_BSDCONFIG is currently required.

Submitted by: Devin Teske (dteske), Ron McDowell <rcm@fuzzwad.org>
Reviewed by: Ron McDowell <rcm@fuzzwad.org>
Approved by: Ed Maste (emaste)