Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Add "ddb capture print" and "ddb capture status" commands do ddb(8),
alowing the DDB output capture buffer to be easily extracted from
user space. Both of these commands include -M/-N arguments, allowing
them to be used with kernel crash dumps (or /dev/mem).

This makes it easier to use DDB scripting and output capture with
minidumps or full dumps rather than with text dumps, allowing DDB
output (scripted or otherwise) to be easily extracted from a crash
dump.

MFC after: 1 week
Discussed with: brooks, jhb

Add command-line tool ddb(8), which allows DDB(4) scripts to be
managed from userspace. It is largely a wrapper for sysctl()
calls, but because the sysctls for adding and removing scripts
are awkward to use directly, this provides an easier-to-use
interface.

MFC after: 3 months