Add const to various variables in the gas sources.

Add const to various variables in the gas sources.

	* symbols.c (decode_local_label_name): Make type a const char *.
	* listing.c (print_source): Make type of p const char *.
	(print_line): Make type of string const	char *.
	(buffer_line): Return const char *.
	(title): Make type const char *.
	(subtitle): Likewise.
	(listing_listing): Make type of p const char *.
	* messages.c (as_internal_value_out_of_range): Make type of prefix
	const char *.
	* stabs.c (s_stab_generic): make type of stab_secname, stabstr_secname
	and string const char *.
	* read.c (_bfd_rel): Make type of name const char *.
	* app.c (out_string): Change type to const char *.
       	(struct app_save::out_string): Likewise.