Implement the V2ClientHello sniff in version-locked methods.

Tested manually by replacing SSLv23_method() with TLSv1_2_method() in
bssl_shim. This is a large chunk of code which is not run in SSLv23_method(),
but it will be run after unification. It's split out separately to ease review.

Change-Id: I6bd241daca17aa0f9b3e36e51864a29755a41097
6 files changed