preserve classes needed for Unix filesystem access from ProGuard

It seems that there are new classes loaded via reflection or JNI in
OpenJDK needed for filesystem access, so we must tell ProGuard to
retain them.

openjdk.pro

@@ -223,3 +223,21 @@
 -keep class sun.util.resources.TimeZoneNames
 -keep class sun.text.resources.FormatData
 
+# loaded via reflection from DefaultFileSystemProvider:
+-keep class sun.nio.fs.LinuxFileSystemProvider
+-keep class sun.nio.fs.BsdFileSystemProvider
+
+# loaded via JNI in UnixNativeDispatcher.c:
+-keep class sun.nio.fs.UnixFileAttributes {
+  <fields>;
+}
+-keep class sun.nio.fs.UnixFileStoreAttributes {
+  <fields>;
+}
+-keep class sun.nio.fs.UnixMountEntry {
+  <fields>;
+}
+
+-keep class sun.nio.fs.UnixException {
+   UnixException(int);
+ }