Command line input handling TLC

I didn't manage to usefully split this further into smaller
independent pieces, so:

 - Use "struct buffer" more.

 - Split out the responsibility of composing a complete command line
   from multiple input lines split with backslash

    (
    E.g.:

       (gdb) print \
       1 + \
       2
       $1 = 3
       (gdb)
    )

   to a separate function.  Note we don't need the separate
   readline_input_state and more_to_come globals at all.  They were
   just obfuscating the logic.

 - Factor out the tricky mostly duplicated code in
   command_line_handler and command_line_input.

gdb/ChangeLog
2016-03-09  Pedro Alves  <palves@redhat.com>

	* event-top.c (more_to_come): Delete.
	(struct readline_input_state): Delete.
	(readline_input_state): Delete.
	(get_command_line_buffer): New function.
	(command_handler): Update comments.  Don't handle NULL commands
	here.  Do not execute commented lines.
	(command_line_append_input_line): New function.
	(handle_line_of_input): New function, partly based on
	command_line_handler and command_line_input.
	(command_line_handler): Rewrite.
	* event-top.h (command_handler): New declaration.
	(command_loop): Defer command execution to command_handler.
	(command_line_input): Update comments.  Simplify, using struct
	buffer and handle_line_of_input.
	* top.h (struct buffer): New forward declaration.
	(handle_line_of_input): New declaration.

gdb/event-top.h

@@ -34,6 +34,8 @@ extern void async_init_signals (void);
 extern void set_async_editing_command (char *args, int from_tty,
 				       struct cmd_list_element *c);
 
+extern void command_handler (char *command);
+
 /* Signal to catch ^Z typed while reading a command: SIGTSTP or SIGCONT.  */
 #ifndef STOP_SIGNAL
 #include <signal.h>