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 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