Enhance the keytool utility to support the storage of passwords in keystores.

The KeyStore API already supports the storage of arbitrary passwords using
Password-based-encryption (PBE) algorithms. Exposing that functionality via keytool
would benefit applications that require a basic credential store. Password retrieval
must be performed programatically.

One simple approach would be to trigger password storage when keystore's existing -genseckey flag is specified with a PBE key algorithm. That would also aid
compatability with earlier JDK releases, if necessary. For example,

    % keytool -genseckey -keyalg PBE

This would then prompt the user for a password and generate a secret key
using the specified PBE algorithm.