Decouple TLS extension table order from tlsext_randomize_build_order()

Decouple TLS extension table order from tlsext_randomize_build_order()

The PSK extension must be the last extension in the client hello. This is
currently implemented by relying on the fact that it is the last extension
in the TLS extension table. Remove this dependency so that we can reorder
the table as needed.

ok tb@