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/core/apparmor-setup.c

@@ -24,7 +24,6 @@ int mac_apparmor_setup(void) {
 #if HAVE_APPARMOR
         _cleanup_(aa_policy_cache_unrefp) aa_policy_cache *policy_cache = NULL;
         _cleanup_(aa_features_unrefp) aa_features *features = NULL;
-        const char *current_file;
         _cleanup_free_ char *current_profile = NULL, *cache_dir_path = NULL;
         int r;