jrtfs supports forwarding to a jrtfs in a target VM

[Sundararajan Athijegannathan]

Hi,

Cross JDK support for jrt file system has been added.

http://hg.openjdk.java.net/jigsaw/jake/jdk/rev/678cec00a4f4
https://bugs.openjdk.java.net/browse/JDK-8066177

With this change, jrt file system supports the following modes:

1. A tool running on JDK 8 can access a JDK 9 run-time image.
2. A tool running on JDK 9 can access its own run-time image.
3. A tool on JDK 9 can access a *different* JDK 9 run-time image - 
perhaps a jlink produced image with fewer modules in it.
Target JDK's home directory is specified via "java.home" property in Map 
passed to FileSystems.newFileSystem(URI, Map) method.
( 
https://docs.oracle.com/javase/8/docs/api/java/nio/file/FileSystems.html#newFileSystem-java.net.URI-java.util.Map- 
).

Thanks,
-Sundar