shared: add root argument to search_and_fopen

This adds the same root argument to search_and_fopen that
conf_files_list already has. Tools that use those two functions as a
pair can now be easily modified to load configuration files from an
alternate root filesystem tree.

src/tmpfiles/tmpfiles.c

@@ -1376,7 +1376,7 @@ static int read_config_file(const char *fn, bool ignore_enoent) {
 
         assert(fn);
 
-        r = search_and_fopen_nulstr(fn, "re", conf_file_dirs, &f);
+        r = search_and_fopen_nulstr(fn, "re", NULL, conf_file_dirs, &f);
         if (r < 0) {
                 if (ignore_enoent && r == -ENOENT)
                         return 0;