(macosx) Applets fail to load when looking for mrj.version

Applets that attempt to access the property mrj.version will get an exception because we haven't added it to the permitted property list.

This is an old property defined by Apple's JDKs, and was used to identify Apple releases of the JDK that didn't coincide with an update to java.version. The typical usage pattern seems to be: "look for os.name and see if it contains 'Mac'. If it's there, get mrj.version".

We should just allow unsigned code to read the property. There's no harm in doing that, and if it's important to the applet they can find a better way to test for compatibility.