When using a key with a hash algorithm, check whether
they can be used together. This should have been happened in the GUI.
In case of an error use the largest algorithm
This fix differs from the proposal in #368 because the signature
of pki_crl::sign() changed after the 2.4.0 release.
However, I was able to increase the bug impact!
Now it crashed instead of only throwing an error! :-)
The signature of the pki_crl::sign() method changed with
commit 61fa66b129, but C++
"outsmarted" me and automatically transformed the EVP_MD == nullptr
into the digest::constructor(EVP_MD) which crashed on the nullptr.
If the digest name is not empty and not understood by openssl
raise an error immediately.
If the nid id NID_undef make sure no openssl-error is stored
in the error list.
Collect all export formats in pki_export.
Each export format has assigned acouple of flags, indicating,
whether they are text, concatenateable, encrypted, usable for
multiple selections or only for a single item.
