Fuzzer mode explores the handshake, but at the cost of losing coverage
on the record layer. Add a separate build flag and client/server
corpora for this mode.

Note this requires tweaks in consumers' fuzzer build definitions.

BUG=111

Change-Id: I1026dc7301645e165a761068a1daad6eedc9271e
Reviewed-on: https://boringssl-review.googlesource.com/12108
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>