- Added support for backreferences in substitution expressions up to nine

 - Added support for backreferences in substitution expressions up to nine
   (\1, \2...\9). This touched a lot of places in this file and I added a new
   function 'print_subst_w_backrefs' in order to keep 'do_subst_command' a
   little more tidy.

	* I tested this good 'n hard, but will always appreciate more testing from
	  other, willing folks.

 - Noticed that the index_of_next_unescaped_slash was subtly wrong so I
   changed both the functionality and behavior (it used to skip over the first
   char in the string you passed it, assuming it was a leading '/'--this
   assumption is no longer made) this necessitated changing the lines that
   call this function just slightly.