Fix nfsdumpstate(8) so that it can print out INET6 callback addresses.

The patch adds support for printing of INET6 callback addresses.
It also adds the #ifdef INET, INET6 as requested by bz@.

PR: 223036
Reviewed by: bz, rgrimes
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19839