add patch that stops screen from writing to a file when the last component

of the filename is  a symbolic link. Note: in some cases the file may still
be opened which can in itself be a problem.  This solves the security
hole where the bad-guy creates a symbolic link named /tmp/screen-exchange
pointing to a file that s/he wishes root to clobber.

misc/screen/patches/patch-af

@@ -0,0 +1,64 @@
+--- fileio.c.~1~	Thu May  1 08:50:25 1997
++++ fileio.c	Mon Jul 27 17:49:58 1998
+@@ -291,6 +291,52 @@
+   DoCommand(args);
+ }
+ 
++/*
++ * Attempt to determine if we're opening the named file or if it
++ * has been replaced by a symbolic link to someplace not nice.
++ * Algorithm is:
++ *
++ *	1) lstat( name );
++ *		if a symbolic link don't even try the open
++ *	2) fd = open( name );
++ *	3) fstat( fd );
++ *	4) lstat( name );
++ *	5) if fstat st_dev and st_ino don't match the lstat values
++ *		then we lost a race and someone added a link between
++ *		the first lstat and the open.  Close the file and
++ *		log an error.  Note: this can still cause problems;
++ *		e.g. a link to /dev/st0 will cause a tape to be
++ *		rewound when the file is closed.
++ */
++static FILE *
++NoLinkOpen( char * fn, char * mode )
++{
++  struct stat linkstat;
++  struct stat filestat;
++  FILE * f;
++
++  if ( lstat( fn, &linkstat ) == 0 &&
++       linkstat.st_mode == S_IFLNK )
++    goto err;
++
++  f = fopen( fn, mode );
++  if ( ! f )
++    return 0;
++
++  if ( fstat( fileno( f ), &filestat ) == 0 &&
++       lstat( fn, &linkstat ) == 0 &&
++       linkstat.st_mode != S_IFLNK &&
++       linkstat.st_dev == filestat.st_dev &&
++       linkstat.st_ino == filestat.st_ino )
++    return f;
++
++  fclose( f );
++
++ err:
++  Msg(0, "Warning: file not opened, \"%s\" is a symbolic link.", fn);
++  return 0;
++}
++
+ void
+ WriteFile(dump)
+ int dump;
+@@ -337,7 +383,7 @@
+   if (UserContext() > 0)
+     {
+       debug("Writefile: usercontext\n");
+-      if ((f = fopen(fn, mode)) == NULL)
++      if ((f = NoLinkOpen(fn, mode)) == NULL)
+ 	{
+ 	  debug2("WriteFile: fopen(%s,\"%s\") failed\n", fn, mode);
+ 	  UserReturn(0);