ClientHello and ServerHello are not allowed to include duplicate extensions.
Add a new helper function to check this and call as appropriate. Remove ad-hoc
per-extension duplicate checks which are no unnecessary.
Add runner.go tests to verify such message correctly rejected.
Change-Id: I7babd5b642dfec941459512869e2dd6de26a831c
Reviewed-on: https://boringssl-review.googlesource.com/1100
Reviewed-by: Adam Langley <agl@google.com>
Initial fork from f2d678e6e89b6508147086610e985d4e8416e867 (1.0.2 beta).
(This change contains substantial changes from the original and
effectively starts a new history.)
