dirmngr: Implement CNAME and SRV record lookup via libdns.
* dirmngr/dns-stuff.c (dns_free): New macro. (libdns): Move var to the top. (libdns_error_to_gpg_error): Map error codes to the new gpg-error codes. (resolve_name_libdns): Restructure code. (getsrv_libdns): New. (get_dns_cname_libdns): New.
- Signed-off-by: Werner Koch <wk@gnupg.org>