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.
