dirmngr: Add a compatibility flag for use with newer Libksba versions.
* dirmngr/dirmngr.h (COMPAT_OCSP_SHA256_CERTID): New. * dirmngr/dirmngr.c (compatibility_flags): Add compat flag. (my_ksba_hash_buffer): Support SHA256. * dirmngr/ocsp.c (do_ocsp_request): Make use of the flag.
Note that this requires a newer Libksba. Older Libksba versions
ignore the request to use SHA256.