Key Agree Update

1. In SendKexDhReply(), break down the key agreement actions for the
   various supported key types into their own functions. Remove
   the redundant variables.
2. In DoKexDhInit(), add flags for the various key agreement types, and
   set them as appropriate when checking the selected kexId. The flags
   are always present no matter the build options.
3. Simplify some of the flag checks for optional options.