RFR 8138766: New default -sigalg for keytool

[Wang Weijun]

Please take a look at

   http://cr.openjdk.java.net/~weijun/8138766/webrev.00/

Now both jarsigner and keytool use AlgorithmId::getDefaultSigAlgForKey to 
decide what the default sigalg should be.

Thanks
Max