1) use `pk_get_rsapubkey` instead of reimplementing the parsing
2) rename the key files, according to their type and key size
3) comment in the data_files/Makefile hoe the keys were generated
4) Fix issue of failure parsing pkcs#1 DER format parsing, missed in previous commit
With SHA-1 deprecation, we need a few certificates using algorithms in
the default support list. Most tests still use SHA-1 though.
The generation process for the new certificates is recorded in the makefile.
