Try path without sbin even if compiled with split-bin=true

I'm working on the transition to merged sbin in Fedora. While the transition is
happening (and probably for a while after), we need to compile systemd with
split-bin=true to support systems upgraded from previous versions. But when the
system has been upgraded and already has /usr/sbin that is a symlink, be nice
and give $PATH without sbin.

We check for both /usr/sbin and /usr/local/sbin. If either exists and is not a
symlink to ./bin, we retain previous behaviour. This means that if both are
converted, we get the same behaviour as split-bin=false, and otherwise we
get the same behaviour as before.

sd-path uses the same logic. This is not a hot path, so I got rid of the nulstr
macros that duplicated the logic.

src/basic/path-util.c

@@ -684,7 +684,7 @@ int find_executable_full(
                  * binary. */
                 p = getenv("PATH");
         if (!p)
-                p = DEFAULT_PATH;
+                p = default_PATH();
 
         if (exec_search_path) {
                 STRV_FOREACH(element, exec_search_path) {
@@ -1441,3 +1441,31 @@ int path_glob_can_match(const char *pattern, const char *prefix, char **ret) {
                 *ret = NULL;
         return false;
 }
+
+const char* default_PATH(void) {
+#if HAVE_SPLIT_BIN
+        static int split = -1;
+        int r;
+
+        /* Check whether /usr/sbin is not a symlink and return the appropriate $PATH.
+         * On error fall back to the safe value with both directories as configured… */
+
+        if (split < 0)
+                STRV_FOREACH_PAIR(bin, sbin, STRV_MAKE("/usr/bin", "/usr/sbin",
+                                                       "/usr/local/bin", "/usr/local/sbin")) {
+                        r = inode_same(*bin, *sbin, AT_NO_AUTOMOUNT);
+                        if (r > 0 || r == -ENOENT)
+                                continue;
+                        if (r < 0)
+                                log_debug_errno(r, "Failed to compare \"%s\" and \"%s\", using compat $PATH: %m",
+                                                *bin, *sbin);
+                        split = true;
+                        break;
+                }
+        if (split < 0)
+                split = false;
+        if (split)
+                return DEFAULT_PATH_WITH_SBIN;
+#endif
+        return DEFAULT_PATH_WITHOUT_SBIN;
+}