secp256k1 for ECDSA signature validation

[bitmarkcc]

With the newer openssl (as in Ubuntu 22) some unit tests fail, so I've implemented Bitcoin's custom library for ECDSA crypto for validating signatures. For new blocks, signatures have to be strictly DER, but for prefork blocks that's not the case, as well as in the unit tests. This library can be useful for other ECDSA crypto operations, but for now we can use it just for the signature validation. Here is the patch: bitmarkcc@e6bdd4b

There are many new files added in the src/secp256k1 dir. You can run a diff and check that they are the same as Bitcoin's (thus safe). The rest of the files changed are configure.ac, src/Makefile.am, src/Makefile.include, src/key.cpp, src/test/Makefile.am.