Cygwin: implement /proc/PID/fd/DESCRIPTOR reopening by handle

Allows expressions along the lines of `cat /proc/self/fd/0 <<EOF'.
The problem here is that the temporary file used for the here script
has already been deleted by the shell.  Opening by filename, as
implemented so far, doesn't work because the file has been moved
to the bin.

Allow reopening files by handle the same way from another process
as long as we have sufficient permissions on the foreign process.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.h

@@ -331,6 +331,7 @@ class fhandler_base
   int open_with_arch (int, mode_t = 0);
   int open_null (int flags);
   virtual int open (int, mode_t);
+  virtual fhandler_base *fd_reopen (int);
   virtual void open_setup (int flags);
   void set_unique_id (int64_t u) { unique_id = u; }
   void set_unique_id () { NtAllocateLocallyUniqueId ((PLUID) &unique_id); }
@@ -2553,6 +2554,7 @@ class fhandler_process: public fhandler_proc
   int closedir (DIR *);
   int __reg3 readdir (DIR *, dirent *);
   int open (int flags, mode_t mode = 0);
+  virtual fhandler_base *fd_reopen (int);
   int __reg2 fstat (struct stat *buf);
   bool fill_filebuf ();