complete DSA support

Complete DSA support:
 - generate parameter & key
 - fix sign/verify
 - remove y (the public key) from the private key, leaving domain parameters
  (i.e., DSA is not RSA, and the exception is RSA).

Note something should be done to handle subprime bits (e.g., 256 in place of 160)
and for SHA2 DSA variants.