RFR 8163256: jlink/plugins/ExcludeVMPluginTest.java failed with Selected VM server doesn't exist

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8163256/webrev.00/ for
https://bugs.openjdk.java.net/browse/JDK-8163256

This is a Mac specific issue. ModuleDescriptor read from
jrt:/java.base/module-info.class resource returns osName() to be
"Darwin" - but, System.getProperty("os.name") still returns "Mac OS X".

For now, I'm fixing ExcludeVMPlugin.java to accept both. With this
change, all jlink jtreg tests pass on Mac.

Thanks,

-Sundar