gdb_readline2 -> gdb_readline_no_editing_callback

gdb_readline2 -> gdb_readline_no_editing_callback

The "2" in "gdb_readline2" doesn't really convey much.  Rename for
clarity.

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

	* event-top.c (gdb_readline2): Rename to ...
	(gdb_readline_no_editing_callback): ... this.
	(change_line_handler, stdin_event_handler)
	(gdb_setup_readline): Adjust.
	* event-top.h (gdb_readline2): Rename to ...
	(gdb_readline_no_editing_callback): ... this, and move closer to
	other readline-related declarations.
	* mi/mi-interp.c (mi_interpreter_resume): Adjust.