dirmngr: Fix recursive resolver mode.

* dirmngr/dns-stuff.c (libdns_init): Initialize options.recurse.

To reproduce an error, run:

./t-dns-stuff --debug --recursive-resolver www.gnupg.org

Then, it returns "No name" error. That's because there was only setup
for root servers, and no setup for recursive query in fact.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>