Move the is_dtls bit from SSL3_ENC_METHOD to SSL_PROTOCOL_METHOD.

This too isn't version-specific. This removes the final difference between TLS
and DTLS SSL3_ENC_METHODs and we can fold them together. (We should be able to
fold away the version-specific differences too, but all in due time.)

Change-Id: I6652d3942a0970273d46d28d7052629c81f848b5
Reviewed-on: https://boringssl-review.googlesource.com/3771
Reviewed-by: Adam Langley <agl@google.com>