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;

src/dissect/dissect.c

@@ -637,7 +637,7 @@ static int parse_argv(int argc, char *argv[]) {
                 }
 
                 case ARG_COPY_OWNERSHIP:
-                        r = parse_tristate_argument("--copy-ownership=", optarg, &arg_copy_ownership);
+                        r = parse_tristate_argument_with_auto("--copy-ownership=", optarg, &arg_copy_ownership);
                         if (r < 0)
                                 return r;
                         break;

src/run/run.c

@@ -1083,7 +1083,7 @@ static int parse_argv_sudo_mode(int argc, char *argv[]) {
                         break;
 
                 case ARG_LIGHTWEIGHT:
-                        r = parse_tristate_argument("--lightweight=", optarg, &arg_lightweight);
+                        r = parse_tristate_argument_with_auto("--lightweight=", optarg, &arg_lightweight);
                         if (r < 0)
                                 return r;
                         break;

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) {

src/shared/parse-argument.h

@@ -4,7 +4,7 @@
 #include "shared-forward.h"
 
 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 parse_json_argument(const char *s, sd_json_format_flags_t *ret);
 int parse_path_argument(const char *path, bool suppress_root, char **arg);
 int parse_signal_argument(const char *s, int *ret);

src/systemctl/systemctl.c

@@ -830,9 +830,9 @@ static int systemctl_parse_argv(int argc, char *argv[]) {
                         break;
 
                 case ARG_CHECK_INHIBITORS:
-                        r = parse_tristate_full(optarg, "auto", &arg_check_inhibitors);
+                        r = parse_tristate_argument_with_auto("--check-inhibitors=", optarg, &arg_check_inhibitors);
                         if (r < 0)
-                                return log_error_errno(r, "Failed to parse --check-inhibitors= argument: %s", optarg);
+                                return r;
                         break;
 
                 case ARG_PLAIN: