mod_compat_roles: Fix attempt to index a nil value #1847 permissions[] is not a map with role names as keys since 817bc9873fc2 but instead a level with host names were added. This was likely an oversight. Refactored towards railroad.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```