print default file paths in named -V

Copying this over from a comment in #858:

It would be nice to be able to get the exact default paths as they are configured in a given BIND build, even if the man page you're looking at was generated elsewhere and doesn't reflect the same configure options for prefix and sysconfdir. This could be added to the output of named -V, for example:

BIND 9.13.5 (Development Release) <id:91bfcfb>
running on Linux x86_64 4.4.0-141-generic #167-Ubuntu SMP Wed Dec 5 10:40:15 UTC 2018
built by make with '--enable-developer'
compiled by GCC 5.4.0 20160609
compiled with OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016
linked to OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016
compiled with libxml2 version: 2.9.3
linked to libxml2 version: 20903
compiled with libjson-c version: 0.11.99
linked to libjson-c version: 0.11.99
compiled with zlib version: 1.2.8
linked to zlib version: 1.2.8
threads support is enabled

default paths:
  named configuration:    /usr/local/etc/named.conf
  rndc configuration:     /usr/local/etc/rndc.conf
  default DNSSEC keys:    /usr/local/etc/bind.keys
  named pid file:         /var/run/named/named.pid
  named lock file:        /var/run/named/named.lock
  named session key file: /var/run/named/session.key