gdb: Pass std::strings to ui_out::field_string () where convenient

While adding a ui_out::text () overload accepting a std::string, I
noticed that several callers of ui_out::field_string () were converting
std::string instances to char pointers even if not necessary.

gdb/ChangeLog:

	* ui-out.c (ui_out::field_string): Add missing style_argument
	to the overload accepting a std::string, to make it equivalent
	to the char pointer version.
	* ui-out.h (class ui_out): Ditto.
	* break-catch-sig.c (signal_catchpoint_print_one): Do not
	convert std::strings to char pointers before passing them to
	ui_out::field_string ().
	* break-catch-throw.c (print_one_detail_exception_catchpoint):
	Ditto.
	* cli/cli-setshow.c (do_show_command): Ditto.
	* disasm.c (gdb_pretty_print_disassembler::pretty_print_insn):
	Ditto.
	* infcmd.c (print_return_value_1): Ditto.
	* inferior.c (print_inferior): Ditto.
	* linux-thread-db.c (info_auto_load_libthread_db): Ditto.
	* mi/mi-cmd-var.c (print_varobj): Ditto.
	(mi_cmd_var_set_format): Ditto.
	(mi_cmd_var_info_type): Ditto.
	(mi_cmd_var_info_expression): Ditto.
	(mi_cmd_var_evaluate_expression): Ditto.
	(mi_cmd_var_assign): Ditto.
	(varobj_update_one): Ditto.
	* mi/mi-main.c (list_available_thread_groups): Ditto.
	(mi_cmd_data_read_memory_bytes): Ditto.
	(mi_cmd_trace_frame_collected): Ditto.
	* osdata.c (info_osdata): Ditto.
	* probe.c (info_probes_for_spops): Ditto.
	* target-connection.c (print_connection): Ditto.
	* thread.c (print_thread_info_1): Ditto.
	* tracepoint.c (print_one_static_tracepoint_marker): Ditto.

gdb/linux-thread-db.c

@@ -1935,7 +1935,7 @@ info_auto_load_libthread_db (const char *args, int from_tty)
 	    i++;
 	  }
 
-	uiout->field_string ("pids", pids.c_str ());
+	uiout->field_string ("pids", pids);
 
 	uiout->text ("\n");
       }