Add chacha aliases for OpenSSL compatibility

Add chacha aliases for OpenSSL compatibility

OpenSSL has the 20 in the long and short names, so add aliases to the
existing names to make things work. In particular, EVP_get_cipherbyname()
will now return EVP_chacha20() for both 'ChaCha20' and 'chacha20'.

Found by Facundo Tuesca when trying to add LibreSSL support for ChaCha20 in
https://github.com/pyca/cryptography/pull/9209

ok jsing