RFR: 8237470: HttpResponse.BodySubscriber::ofFile throws UOE with non-default file systems

[Julia Boes]

Hi,

We recently added support for non-default file systems to the HttpClient 
on the sending side. This fix expands the support to the receiving side.

More specifically, the change is in ResponseBodyHandlers.PathBodyHandler 
and ResponseSubscribers.PathSubscriber. In both cases, an 
UnsupportedOperationException is now caught if a java.io.File cannot be 
obtained and an AccessControlContext is captured to open a FileChannel 
with a privileged scope.

ResponseBodyHandlers.FileDownloadBodyHandler is changed to consistently 
throw IllegalArgumentException if a path of a non-default file system is 
passed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237470

CSR (draft): https://bugs.openjdk.java.net/browse/JDK-8245770

Webrev: http://cr.openjdk.java.net/~jboes/webrevs/8237470/webrev.00/

Tests were run with repeat 50 cross-platform.

Regards,

Julia