Support ciphersuites with matching mac/cipher(/KX)

GOST TLS 1.3 will have two pairs of ciphersuites, each pair using the same cipher and MAC (differing only in external re-keying settings). GnuTLS can not handle this at this moment when generating priority lists.

Another use case is supporting ciphersuites during standardisation phase, when one would be using both assigned and private ids.

For my tests I have been extending set_cipher_list() with if ce->id == ID1 then also add ciphersuite ID2, but this does not look like a production code.