dbusbind: Add alternative form of compound argument

dbusbind: Add alternative form of compound argument

* src/dbusbind.c (xd_append_basic_arg): New function, split from
xd_append_arg.
(xd_append_arg): Use xd_append_arg.
(xd_type_spec_to_signature): New function.
(xd_append_arg_with_type_spec): New function.
(Fdbus_message_internal): Use xd_append_arg_with_type_spec,
instead of xd_append_arg.
(syms_of_dbusbind): Provide subfeature `:type'.
* doc/misc/dbus.texi (Type Conversion): Mention `:type' keyword.

Fixes: debbugs:20193