strv: declare iterator of FOREACH_STRING() in the loop

Same idea as 03677889f0.

No functional change intended. The type of the iterator is generally changed to
be 'const char*' instead of 'char*'. Despite the type commonly used, modifying
the string was not allowed.

I adjusted the naming of some short variables for clarity and reduced the scope
of some variable declarations in code that was being touched anyway.

src/test/test-loop-block.c

@@ -114,7 +114,6 @@ int main(int argc, char *argv[]) {
         _cleanup_(dissected_image_unrefp) DissectedImage *dissected = NULL;
         _cleanup_(umount_and_rmdir_and_freep) char *mounted = NULL;
         pthread_t threads[N_THREADS];
-        const char *fs;
         sd_id128_t id;
         int r;