Make EVP_CIPHER_CTX_copy work in GCM mode.

PR#3272
More constification.
Clarified that the signature's buffer size, `s`, is not used as an
IN parameter.

Under the old docs, the only thing stated was "at most
EVP_PKEY_size(pkey) bytes will be written". It was kind of misleading
since it appears EVP_PKEY_size(pkey) WILL be written regardless of the
signature's buffer size.
Don't core dump when using CMAC with dgst.

We can't unfortunately print the CMAC cipher used without extending the API.

PR#2579