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/shared/parse-argument.c

@@ -20,10 +20,12 @@ int parse_boolean_argument(const char *optname, const char *s, bool *ret) {
 
         /* Returns the result through *ret and the return value. */
 
+        assert(optname);
+
         if (s) {
                 r = parse_boolean(s);
                 if (r < 0)
-                        return log_error_errno(r, "Failed to parse boolean argument to %s: %s.", optname, s);
+                        return log_error_errno(r, "Failed to parse boolean argument to '%s': %s", optname, s);
 
                 if (ret)
                         *ret = r;
@@ -36,24 +38,20 @@ int parse_boolean_argument(const char *optname, const char *s, bool *ret) {
         }
 }
 
-int parse_tristate_argument(const char *optname, const char *s, int *ret) {
+int parse_tristate_argument_with_auto(const char *optname, const char *s, int *ret) {
         int r;
 
-        if (s) {
-                r = parse_boolean(s);
-                if (r < 0)
-                        return log_error_errno(r, "Failed to parse boolean argument to %s: %s.", optname, s);
+        assert(optname);
+        assert(s); /* We refuse NULL optarg here, since that would be ambiguous on cmdline:
+                      for --enable-a[=BOOL], --enable-a is intuitively interpreted as true rather than "auto"
+                      (parse_boolean_argument() does exactly that). IOW, tristate options should require
+                      arguments. */
 
-                if (ret)
-                        *ret = r;
+        r = parse_tristate_full(s, "auto", ret);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse tristate argument to '%s': %s", optname, s);
 
-                return r;
-        } else {
-                if (ret)
-                        *ret = -1;
-
-                return 0;
-        }
+        return 0;
 }
 
 int parse_json_argument(const char *s, sd_json_format_flags_t *ret) {