Add SSL_get_rc4_state.

This allows the current RC4 state of an SSL* to be extracted. We have
internal uses for this functionality.

Change-Id: Ic124c4b253c8325751f49e7a4c021768620ea4b7
Reviewed-on: https://boringssl-review.googlesource.com/3722
Reviewed-by: Adam Langley <agl@google.com>