MFC r314654:

cddl: normalize paths using SRCTOP-relative paths or :H when possible

This simplifies make logic/output

While here, remove bogus CFLAGS which look for headers in cddl/lib/libumem.
There aren't any source files there (just Makefiles)

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

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

Add a man page for plockstat.

Sponsored by: The FreeBSD Foundation

Port plockstat to FreeBSD. This will be connected to the build later.

Sponsored by: The FreeBSD Foundation