gdb: make add_setshow commands return set_show_commands

gdb: make add_setshow commands return set_show_commands

Some add_set_show commands return a single cmd_list_element, the one for
the "set" command.  A subsequent patch will need to access the show
command's cmd_list_element as well.  Change these functions to return a
new structure type that holds both pointers.

I initially only modified add_setshow_boolean_cmd (the one I needed),
but I think it's better to change the whole chain to keep everything in
sync.

gdb/ChangeLog:

	* command.h (set_show_commands): New.
	(add_setshow_enum_cmd, add_setshow_auto_boolean_cmd,
	add_setshow_boolean_cmd, add_setshow_filename_cmd,
	add_setshow_string_cmd, add_setshow_string_noescape_cmd,
	add_setshow_optional_filename_cmd, add_setshow_integer_cmd,
	add_setshow_uinteger_cmd, add_setshow_zinteger_cmd,
	add_setshow_zuinteger_cmd, add_setshow_zuinteger_unlimited_cmd):
	Return set_show_commands.  Adjust callers.
	* cli/cli-decode.c (add_setshow_cmd_full): Return
	set_show_commands, remove result parameters, adjust callers.

Change-Id: I17492b01b76002d09effc84830f9c6db26f1db7a