* dtable.cc (dtable::build_fhandler): Accept an optional path_conv argument.

If available, use this to calculate path name and device number.
* dtable.h (dtable): Reflect above change.
* fhandler.h (fhandler_base): Declare virtual method which accepts path_conv
rather than path string as first argument.
* fhandler.cc (fhandler_base::open): Define above new method.
* syscalls.cc (_open): Set aside a path_conv variable for use in build_fhandler
and subsequent call to open.

winsup/cygwin/fhandler.h

@@ -320,6 +320,7 @@ public:
   {
     return open (flags, mode);
   }
+  virtual int open (path_conv& real_path, int flags, mode_t mode);
   virtual int open (int flags, mode_t mode = 0);
   virtual int close ();
   virtual int fstat (struct stat *buf) { return stat_dev (get_device (), get_unit (), get_namehash (), buf); }