udev: use the usual set of load paths for udev rules

This adds /usr/local/lib/udev/rules.d to the search path on non-split-usr systems.
On split-usr systems, the paths with /usr/-prefixes are added too.
In the past, on split-usr systems, it made sense to only load rules from
/lib/udev/rules.d, because /usr could be mounted late. But we don't support running
without /usr since 80758717a6, so in practice it doesn't matter whether the
rules files are in /lib/udev/rules.d or /usr/lib/udev/rules.d. Distributions
that maintain the illusion of functional split-usr are welcome to simply not put any
files in /usr/lib/udev/rules.d/.

In practice this doesn't change much, but it makes udev more consistent with the
rest of the systemd suite.

src/udev/udev-rules.c

@@ -15,6 +15,7 @@
 
 #include "alloc-util.h"
 #include "conf-files.h"
+#include "def.h"
 #include "device-private.h"
 #include "device-util.h"
 #include "dirent-util.h"
@@ -41,6 +42,7 @@
 #include "util.h"
 
 #define PREALLOC_TOKEN          2048
+#define RULES_DIRS (const char* const*) CONF_PATHS_STRV("udev/rules.d")
 
 struct uid_gid {
         unsigned name_off;
@@ -50,13 +52,6 @@ struct uid_gid {
         };
 };
 
-static const char* const rules_dirs[] = {
-        "/etc/udev/rules.d",
-        "/run/udev/rules.d",
-        UDEVLIBEXECDIR "/rules.d",
-        NULL
-};
-
 struct UdevRules {
         usec_t dirs_ts_usec;
         ResolveNameTiming resolve_name_timing;
@@ -1609,7 +1604,7 @@ int udev_rules_new(UdevRules **ret_rules, ResolveNameTiming resolve_name_timing)
 
         udev_rules_check_timestamp(rules);
 
-        r = conf_files_list_strv(&files, ".rules", NULL, 0, rules_dirs);
+        r = conf_files_list_strv(&files, ".rules", NULL, 0, RULES_DIRS);
         if (r < 0)
                 return log_error_errno(r, "Failed to enumerate rules files: %m");
 
@@ -1661,7 +1656,7 @@ bool udev_rules_check_timestamp(UdevRules *rules) {
         if (!rules)
                 return false;
 
-        return paths_check_timestamp(rules_dirs, &rules->dirs_ts_usec, true);
+        return paths_check_timestamp(RULES_DIRS, &rules->dirs_ts_usec, true);
 }
 
 static bool match_key(UdevRules *rules, struct token *token, const char *val) {