Split tls1_check_ec_key.

This avoids the strange optional parameter thing by moving it to the client.
Also document what the functions should do.

Change-Id: I361266acadedfd2bfc4731f0900821fc2c2f954d
Reviewed-on: https://boringssl-review.googlesource.com/1843
Reviewed-by: Adam Langley <agl@google.com>