Clean up and refactor server side DHE key exchange.

Clean up and refactor server side DHE key exchange.

Provide ssl_kex_generate_dhe_params_auto() which handles DHE key generation
based on parameters determined by the specified key bits. Convert the
existing DHE auto parameter selection code into a function that just tells
us how many key bits to use.

Untangle and rework the server side DHE key exchange to use the ssl_kex_*
functions.

ok inoguchi@ tb@