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

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

Add libnetbsd, a thin compatibility layer intended to allow a limited
set of NetBSD software to compile as part of the FreeBSD build with
little or no modifiction. It is built as a static library and not
installed for general use. Likewise, its header files are not
installed.

Sponsored by: DARPA, AFRL