&do_cmd_rfc: New function. Handles \rfc{}.

$STRIP_INDEX_TT:  New flag.  If set, the <tt>...</tt> around stuff in the index
	is dropped.  This is more O'Reilly-like.

&make_str_index_entry:  Honor $STRIP_INDEX_TT.

&make_mod_index_entry:  Honor $STRIP_INDEX_TT.