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: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2156)

crypto/evp/e_chacha20_poly1305.c

@@ -127,7 +127,7 @@ static const EVP_CIPHER chacha20 = {
     1,                      /* block_size */
     CHACHA_KEY_SIZE,        /* key_len */
     CHACHA_CTR_SIZE,        /* iv_len, 128-bit counter in the context */
-    0,                      /* flags */
+    EVP_CIPH_CUSTOM_IV | EVP_CIPH_ALWAYS_CALL_INIT,
     chacha_init_key,
     chacha_cipher,
     NULL,