dirmngr: Implement querying nameservers over IPv6.
* dirmngr/dns.c (dns_so_check): Reinitialize sockets on address family mismatch. (enum dns_res_state): New states for querying over IPv6. (dns_res_exec): Implement the new states by copying and modifying the IPv4 variants. Branch to their respective counterparts if the current list of resolvers using the current address family is exhausted.
This allows dirmngr to resolve names on systems where the nameservers
are only reachable via IPv6.
- GnuPG-bug-id: T2990
- Signed-off-by: Justus Winter <firstname.lastname@example.org>