Promote max_cert_list and max_send_fragment to functions.

Also size them based on the limits in the quantities they control (after
checking bounds at the API boundary).

BUG=404754

Change-Id: Id56ba45465a473a1a793244904310ef747f29b63
Reviewed-on: https://boringssl-review.googlesource.com/4559
Reviewed-by: Adam Langley <agl@google.com>