Key Agree Update

1. In the key signature block, add flag for Ecc, and for the key
   allocation.
2. Add FreePubKey() to delete the pub key stored in the key signature
   block.
3. In DoKexDhReply(), break down the key agreement actions for the
   various supported key types into their own functions. Remove the
   redundant variables.
4. Using flags that are always present, reduce some of the complicated
   flag checks.
5. Fix a compile guard where the ECDH private key used by the client is
   disabled by ECDSA.