gdb/guile: use return values of add_setshow functions in add_setshow_generic

gdb/guile: use return values of add_setshow functions in add_setshow_generic

Use the set_show_commands objects returned by the add_setshow functions
in add_setshow_generic.  This lets us avoid looking up the commands
after creating them, instead using the return objects directly.

Make add_setshow_generic return a set_show_commands object, which is a
bit nicer than returning both commands by parameter.

Finally, store using that object in param_smob.

Equivalent of 7bd22f56a3cf ("gdb/python: use return values of
add_setshow functions in add_setshow_generic"), but for guile.

gdb/ChangeLog:

	* guile/scm-param.c (struct param_smob) <set_command,
	show_command>: Remove.
	<commands>: New.
	(pascm_is_valid): Adjust.
	(add_setshow_generic): Use return values of add_setshow
	functions, return a set_show_commands.
	(gdbscm_register_parameter_x): Adjust.

Change-Id: I18ed9e7dd5646529491c86749a5cb20763acd1f0