dirmngr: Keep track of domains used for WKD queries

* dirmngr/domaininfo.c: New file.
* dirmngr/Makefile.am (dirmngr_SOURCES): Add file.
* dirmngr/server.c (cmd_wkd_get): Check whether the domain is already
known and tell domaininfo about the results.

This adds a registry for domain information to eventually avoid
useless queries for domains which do not support WKD. The missing
part is a background task to check whether a queried domain supports
WKD at all and to expire old entries.

• Signed-off-by: Werner Koch <wk@gnupg.org>