RFR: 8151893: Add security property to configure XML Signature secure validation mode

[Sean Mullan]

Please review this fix to add a new security property that allows you to 
configure the individual restrictions that are enabled by the XML 
Signature secure validation mode.

bug: https://bugs.openjdk.java.net/browse/JDK-8151893
webrev: http://cr.openjdk.java.net/~mullan/webrevs/8151893/webrev.00/
Thanks,
Sean