Cygwin: introduce fhandler_process_fd and add stat(2) handling

move special fd symlink code into own fhandler_process_fd class
to simplify further additions to /proc/PID/fd/DESCRIPTOR symlink
handling.

Add a method to handle stat(2) on such a proc fd symlink by handle.
This allows correct reply from stat(2) if the target file has been
deleted.  This eventually fixes `awk -f /dev/fd/3 3<<eof'.

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

winsup/cygwin/syscalls.cc

@@ -2005,7 +2005,8 @@ extern "C" int
 stat64 (const char *__restrict name, struct stat *__restrict buf)
 {
   syscall_printf ("entering");
-  path_conv pc (name, PC_SYM_FOLLOW | PC_POSIX | PC_KEEP_HANDLE,
+  path_conv pc (name, PC_SYM_FOLLOW | PC_POSIX | PC_KEEP_HANDLE
+		      | PC_SYM_NOFOLLOW_PROCFD,
 		stat_suffixes);
   return stat_worker (pc, buf);
 }