parse-argument: make parse_tristate_argument() do something useful

I expressed the issue I have with parse_tristate_argument()
in #37751: it doesn't add any value to direct use of parse_tristate();
on the contrary, it doesn't support means to reset the arg to "auto"/-1 state.
The mere reason it existed is that we need a int type ret param.

Since the previous attempt to address this mess failed, let's
try to make the function more useful by making it accept "auto".
I figure this is useful on its own.

As requested in
https://github.com/systemd/systemd/pull/40652#discussion_r2831833996,
the function name is suffixed with _with_auto() to establish
that "auto" is handled internally.

src/bootctl/bootctl.c

@@ -521,7 +521,7 @@ static int parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_VARIABLES:
-                        r = parse_tristate_argument("--variables=", optarg, &arg_touch_variables);
+                        r = parse_tristate_argument_with_auto("--variables=", optarg, &arg_touch_variables);
                         if (r < 0)
                                 return r;
                         break;