Add missing flags for EVP_chacha20()

ChaCha20 code uses its own custom cipher_data. Add EVP_CIPH_CUSTOM_IV
and EVP_CIPH_ALWAYS_CALL_INIT so that the key and the iv can be set by
different calls of EVP_CipherInit_ex().

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2156)
(cherry picked from commit c83680a0)