Fix nfsuserd so that it handles the mapped localhost address when jails
are enabled.

The nfsuserd(8) daemon does not function correctly when jails are enabled,
since localhost gets mapped to another IP address and, as such, the upcall
RPC fails.
This patch fixes the problem by doing a getsockname(2) of a socket mapped
to localhost to find out what the correct address is for the comparison
test with the upcall's from IP address.
This patch also adds INET6 support and the required #ifdef's for INET and
INET6. It now uses INET6 by default for the upcalls, if the kernel has
INET6 support and the daemon is also built with INET6 support.

Tested by: freebsd@danielengel.com (earlier version)
PR: 205193
Reviewed by: bz, rgrimes
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19218