MFC: r345992, r346087
Add INET6 support for the upcalls to the nfsuserd daemon.

The kernel code uses UDP to do upcalls to the nfsuserd(8) daemon to get
updates to the username<->uid and groupname<->gid mappings.
A change to AF_LOCAL last year had to be reverted, since it could result
in vnode locking issues on the AF_LOCAL socket.
This patch adds INET6 support and the required #ifdef INET and INET6
to the code.

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

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

Fix make buildworld -DMODULES_WITH_WORLD
Sort opt_ srcs

Add the kernel build glue for the experimental NFS subsystem that
includes support for NFSv4. The subsystem can optionally be linked
into the kernel using the two options:
NFSCL - the client
NFSD - the server
It is also built as three modules:
nfscl - the client
nfsd - the server
nfscommon - functions shared by the client and server

Approved by: kib (mentor)