strv: make iterator in STRV_FOREACH() declaread in the loop

This also avoids multiple evaluations in STRV_FOREACH_BACKWARDS()

src/basic/fileio.c

@@ -1042,8 +1042,6 @@ static int search_and_fopen_internal(
                 FILE **ret,
                 char **ret_path) {
 
-        char **i;
-
         assert(path);
         assert(mode);
         assert(ret);