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

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

Introduce two new options MK_INET and MK_INET_SUPPORT analogically
with INET6 equivalents. Patch reather than re-genenerating src.conf
(given the current problem with the script that does the re-gen).

Reviewed by: gnn
Sponsored by: The FreeBSD Foundation
Sponsored by: iXsystems
MFC after: 2 weeks