Cygwin: proc fd: fix handling of pipes, sockets, etc

The symlink target of /proc/PID/fd files pointing to pipes and
sockets are just artificial filenames referencing the object using
some internal number.  The pipe open code expects a path specifying
process pid and the internal number so it access the right process
and pipe.

- Set the posix path of the pipe to the simple pipe name only,
  as it shows up in /proc/PID/fd.  A /proc/self prefix is just
  as wrong as a /dev/fd prefix.

- Revert thinko in fhandler_pipe::open expecting the name as
  /proc/self/fd/...  In fact this should never happen.

- Fix up the path before re-opening the pipe instead.

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

winsup/cygwin/fhandler_pipe.cc

@@ -73,11 +73,8 @@ fhandler_pipe::open (int flags, mode_t mode)
   bool inh;
   bool got_one = false;
 
-  if (sscanf (get_name (), "/proc/self/fd/pipe:[%llu]",
-	      (long long *) &uniq_id) == 1)
-    pid = myself->pid;
-  else if (sscanf (get_name (), "/proc/%d/fd/pipe:[%llu]",
-		   &pid, (long long *) &uniq_id) < 2)
+  if (sscanf (get_name (), "/proc/%d/fd/pipe:[%llu]",
+	      &pid, (long long *) &uniq_id) < 2)
     {
       set_errno (ENOENT);
       return 0;