dirmngr: Improve finding OCSP cert.

* dirmngr/certcache.c (find_cert_bysubject): Add better debug output
and try to locate by keyid.

This change was suggested in T4536
but we do not have any test cases for this.

(cherry picked from commit 4699e294cc9e59f35262adca26ca291927acca9e)

The bug report meanwhile has a test description but I have not done
the testing yet. I port this back to 2.2 anyway given that no
regression have been reported for master in nearly a year.

• GnuPG-bug-id: T4536
• Signed-off-by: Werner Koch <wk@gnupg.org>