Also fix some DTLS cookie bugs. rcvd_cookie is never referenced after being
saved (and the length isn't saved, so it couldn't be used anyway), and the
cookie verification failed to check the length.

For convenience, add a CBS_mem_equal helper function. Saves a bit of
repetition.

Change-Id: I187137733b069f0ac8d8b1bf151eeb80d388b971
Reviewed-on: https://boringssl-review.googlesource.com/1174
Reviewed-by: Adam Langley <agl@google.com>

Change-Id: I673c929b78bcf6952db8dfb295dd79d455bcb2a0
Reviewed-on: https://boringssl-review.googlesource.com/1070
Reviewed-by: Adam Langley <agl@google.com>

This gives us systematic bounds-checking on all the parses. Also adds a
convenience function, CBS_memdup, for saving the current contents of a CBS.

Change-Id: I17dad74575f03121aee3f771037b8806ff99d0c3
Reviewed-on: https://boringssl-review.googlesource.com/1031
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)