RFR: 8176709: JarFileSystem::isMultiReleaseJar is incorrect

[Claes Redestad]

Hi,

please review this change to adapt the JarFileSystem::isMultiReleaseJar
method to align with the evolved specification in JEP 238

Bug: https://bugs.openjdk.java.net/browse/JDK-8176709
Webrev: http://cr.openjdk.java.net/~redestad/8176709/jdk.01/

This unfortunately adds a qualified export from java.base to jdk.zipfs,
but since the jdk.internal.util.jar package was already exported to
three other modules I think it's a low cost option that is preferable
to other alternatives such as maintaining separate implementations.

Thanks!

/Claes