RFR 8057810: New defaults for DSA keys in jarsigner and keytool

[Wang Weijun]

Hi All

Please review the code change at

   http://cr.openjdk.java.net/~weijun/8057810/webrev.00/

Now DSA keys use 2048 bit key size and SHA256withDSA as defaults for keytool 
and jarsigner.

A new test added. Several existing tests updated since the new defaults are not 
supported by NSS.

Thanks
Max