Align RSA and EC key generation with each other

Align RSA and EC key generation with each other

Being two different cryptographic primitives, it is clear that there must
be some differences between RSA and EC keygen, but they don't have to be
entirely different. We need to set the key type, RSA needs a bit size and
ECDSA needs a curve. That's all the differences there need to be.

Garbage collect a few useless elses and avoid two exit labels paths where
one would do just fine.

As another small bonus, this file no longer uses "deprecated API", so the
portable fork can get rid of an ugly openssl 3 patch if they want to.

ok florian