dirmngr: Handle failures related to missing IPv6 gracefully

* dirmngr/ks-engine-hkp.c (handle_send_request_error): Handle two more
error codes.

Handle the two possible connect failures related to missing IPv6 support
gracefully by marking the host dead and retrying with another one.
If IPv6 is disabled via procfs, connect() will return EADDRNOTAVAIL.
If IPv6 is not compiled into the kernel, it will return EAFNOSUPPORT.
This makes it possible to use dual-stack hkp servers on hosts not having
IPv6 without random connection failures.

The above description seems to be for Linux, so it is possible that
other systems might behave different. However, it is worth to try
this patch.

• GnuPG-bug-id: T3331
• Signed-off-by: Werner Koch <wk@gnupg.org>