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-libcrypt-util.c

@@ -39,7 +39,6 @@ static int test_hash_password(void) {
         /* As a warm-up exercise, check if we can hash passwords. */
 
         bool have_sane_hash = false;
-        const char *hash;
 
         FOREACH_STRING(hash,
                        "ew3bU1.hoKk4o",
@@ -68,7 +67,6 @@ static void test_hash_password_full(void) {
         log_info("/* %s */", __func__);
 
         _cleanup_free_ void *cd_data = NULL;
-        const char *i;
         int cd_size = 0;
 
         log_info("sizeof(struct crypt_data): %zu bytes", sizeof(struct crypt_data));