gpgsm: Handle re-issued CA certificates in a better way.

* sm/certchain.c (find_up_search_by_keyid): Consider all matching
certificates.
(find_up): Add some debug messages.

The DFN-Verein recently re-issued its CA certificates without
generating new keys. Thus looking up the chain using the authority
keyids works but may use still existing old certificates. This may
break the CRL lookup in the Dirmngr. The hack to fix this is by using
the latest issued certificate with the same subject key identifier.

As usual Peter Gutman's X.509 style guide has some comments on that
re-issuing.

• GnuPG-bug-id: T1644