Fix bug in handling the state BOM marker and also extend to work

composing external formats.  The state BOM marker also needs to
indicate how long the BOM is.

 * src/code/stream.lisp
   * Handle the BOM marker correctly for composed format. 
   * Handle the new values for the state BOM marker.

 * src/pcl/simple-streams/external-formats/utf-16.lisp
   * Change BOM state marker to +2 and -2 instead of 1 and 2 to
     indicate the length of the BOM.

 * src/pcl/simple-streams/external-formats/utf-32.lisp
   * Change BOM state marker to +4 and -4 instead of 1 and 2 to
     indicate the length of the BOM.