Add new port dns/powerdns-recursor40

FreeBSD 10.3 and below doesn't support __cxa_thread_atexit() in
libc, which is required for building PowerDNS Recursor 4.1.0.

This new port follows the 4.0.x branch, which has no such
requirement.

PR: 224297
Submitted by: tremere@cainites.net
Approved by: mat
Differential Revision: https://reviews.freebsd.org/D13540