dirmngr: Support rsaPSS also in the general validate module.

* dirmngr/validate.c (hash_algo_from_buffer): New.
(uint_from_buffer): New.
(check_cert_sig): Support rsaPSS.
* sm/certcheck.c (gpgsm_check_cert_sig): Fix small memory leak on
error.

Yes, I know that there is a lot of code duplication. In fact some of
the code is ugly and it would be better if we enhance Libgcrypt to
guarantee that returned memory buffers via gcry_sexp_extract_param are
allways Nul terminated and we should also enhance that function to
directly extract into an unsigned int or char *.

• GnuPG-bug-id: T4538
• Signed-off-by: Werner Koch <wk@gnupg.org>