libdns: Provide replacement for EPROTO.
* dirmngr/dns.c (EPROTO) ![EPROTO]: Define to EPROTONOSUPPORT.
This is the same replacement we use in Libassuan
(commit 8ab3b9273524bd344bdb90dd5d3bc8e5f53ead6e) to make it work on
OpenBSD and may other BSD based OSes.
- Signed-off-by: Werner Koch <wk@gnupg.org>