This avoids duplicating the code to build the final premaster in PSK and
ECDHE_PSK. It also ports it to CBB for an initial trial of the API. Computing
the premaster secret now proceeds in four steps:

1. If a PSK key exchange (alg_a), look up the pre-shared key.
2. Compute the premaster secret based on alg_k. If PSK, it's all zeros.
3. If a PSK key exchange (alg_a), wrap the premaster in a struct with the
   pre-shared key.
4. Use the possibly modified premaster to compute the master secret.

Change-Id: Ib511dd2724cbed42c82b82a676f641114cec5470
Reviewed-on: https://boringssl-review.googlesource.com/1173
Reviewed-by: Adam Langley <agl@google.com>