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/sysctl/sysctl.c

@@ -123,7 +123,7 @@ static int parse_file(Hashmap *sysctl_options, const char *path, bool ignore_eno
 
         assert(path);
 
-        r = search_and_fopen_nulstr(path, "re", conf_file_dirs, &f);
+        r = search_and_fopen_nulstr(path, "re", NULL, conf_file_dirs, &f);
         if (r < 0) {
                 if (ignore_enoent && r == -ENOENT)
                         return 0;