(Vthis_command): Renamed from this_command.

(Vthis_command): Renamed from this_command.
(real_this_command): New variable, but not a Lisp variable;
updated like Vthis_command, but never altered by Lisp programs.
(command_loop_1): Use real_this_command to set Vreal_last_command.

(read_char): When input method returns no chars, call cancel_echoing.
Restore the previous echo area message and this_command_keys, too.

(Vinput_method_previous_message): New variable.
(syms_of_keyboard): Set up lisp variable.
(Qinput_method_exit_on_first_char, Qinput_method_use_echo_area): New variables.
(syms_of_keyboard): Initialize them.
(Fread_key_sequence): New arg COMMAND_LOOP.
Bind those variables.