sysusers: allow overrides in /etc and /run

An administrator might want to block a certain sysusers config file from
being executed, e.g. to block the creation of a certain user.

Only a relatively short description is added in the man page, since
overrides should be relatively rare.

src/sysusers/sysusers.c

@@ -62,6 +62,8 @@ typedef struct Item {
 static char *arg_root = NULL;
 
 static const char conf_file_dirs[] =
+        "/etc/sysusers.d\0"
+        "/run/sysusers.d\0"
         "/usr/local/lib/sysusers.d\0"
         "/usr/lib/sysusers.d\0"
 #ifdef HAVE_SPLIT_USR