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.

Requested by: bz
PR: 205193
Reviewed by: bz, rgrimes
MFC after: 2 weeks
Differential Revision: http://reviews.freebsd.org/D19218