JDK-8232280: close() is not called on return value of Files.walk()

[Andy Herrick]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

Fix is to wrap all Files.walk() calls in try-with-resource blocks.

[1] https://bugs.openjdk.java.net/browse/JDK-8232280

[2] http://cr.openjdk.java.net/~herrick/8232280/webrev.01/

/Andy