We do this enough that it's worth extracting a common parser. And this
gives a struct we can pass around. Note this moves the server extensions
block parsing out of ssl_scan_serverhello_tlsext.
I've also consolidated a few error conditions to tighten the code up a
bit: the TLS 1.2 code distinguishes unknown from unadvertised cipher,
while the TLS 1.3 code didn't. And seeing the wrong legacy version
number in TLS 1.3 is really just a syntax error since it's not the
version field anymore. (RFC8446 specifies the value.)
Change-Id: Ia2f44ff9a3899b5a594569f1b258f2b487930496
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48908
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
David Benjamin