Merge pull request #13485 from rgacogne/directory-visitor

Add `pdns::visit_directory()`, wrapping opendir/readdir/closedir

diff --cc pdns/arguments.cc
Simple merge

diff --cc pdns/dnsbackend.cc
Simple merge

diff --cc pdns/ixfrdist.cc
Simple merge

diff --cc pdns/ixfrutils.cc
Simple merge