Address some warnings.
Nettle: Use cpp to remove some dead code in the ILP32 case.

This section of code isn't reached in the ILP32 case, as
asize then can't reach above 0xffffffffUL which is needed
to reach the code.

Also reorders the code for easier use of cpp.
Code reuse
Testsuite [ssl]: Test the negotiated protocol version.

Test cipher-suite driven protocol downgrade for all client/server
combinations.

Test that the negotiated SSL/TLS version is the expected version.