Remove cmd_cfunc_ftype

Remove cmd_cfunc_ftype

This removes cmd_cfunc_ftype and the non-const overload of add_cmd;
then fixes up the fallout.

For the most part this patch is straightforward.  There are a few
files (go32-nat.c, windows-nat.c, and gnu-nat.c) that I could not
compile; so I made a best effort there.

gdb/ChangeLog
2017-11-07  Tom Tromey  <tom@tromey.com>

	* go32-nat.c (go32_sysinfo, go32_sldt, go32_sgdt, go32_sidt)
	(go32_pde, go32_pte, go32_pte_for_address): Constify.
	* gnu-nat.c (_parse_bool_arg, show_thread_default_pause_cmd)
	(set_thread_default_pause_cmd, set_thread_default_run_cmd)
	(show_thread_default_run_cmd, set_thread_default_detach_sc_cmd)
	(parse_int_arg, show_thread_default_detach_sc_cmd)
	(set_signals_cmd, show_signals_cmd, set_sig_thread_cmd)
	(show_sig_thread_cmd, set_stopped_cmd, show_stopped_cmd)
	(set_exceptions_cmd, show_exceptions_cmd, set_task_pause_cmd)
	(show_task_pause_cmd, set_task_detach_sc_cmd)
	(show_task_detach_sc_cmd, set_task_exc_port_cmd)
	(set_noninvasive_cmd, set_thread_pause_cmd)
	(show_thread_pause_cmd, set_thread_run_cmd, show_thread_run_cmd)
	(set_thread_detach_sc_cmd, show_thread_detach_sc_cmd)
	(set_thread_exc_port_cmd, thread_takeover_sc_cmd): Constify.
	* windows-nat.c (display_selectors): Constify.
	* cli/cli-decode.h (struct cmd_list_element) <function>: Remove
	non-const "cfunc".
	* cli/cli-decode.c (set_cmd_cfunc): Remove non-const overload.
	(cmd_cfunc_eq): Likewise.
	(struct cmd_list_element): Likewise.
	(do_cfunc): Remove.
	(cli_user_command_p): Update.
	* command.h (add_cmd): Remove non-const overload.
	(cmd_cfunc_ftype): Remove typedef.
	(cmd_cfunc_eq): Remove non-const overload.
	* value.c (show_values): Constify.
	* thread.c (thread_apply_all_command): Constify.
	* symfile.c (load_command): Constify.
	* source.c (directory_command): Constify.
	* maint.c (maintenance_internal_error)
	(maintenance_demangler_warning, maintenance_space_display)
	(maintenance_print_architecture, maintenance_translate_address)
	(maintenance_info_selftests, maintenance_internal_warning):
	Constify.
	* breakpoint.c (disable_trace_command, enable_trace_command):
	Constify.
	* auto-load.c (info_auto_load_local_gdbinit, add_auto_load_dir):
	Constify.
	(add_auto_load_safe_path): Constify.
	* guile/scm-auto-load.c (info_auto_load_guile_scripts): Constify.
	* top.h (show_commands): Constify.
	* linux-thread-db.c (info_auto_load_libthread_db): Constify.
	* sparc64-tdep.c (adi_examine_command): Constify.
	(adi_assign_command): Constify.