lualoader: provide module-manipulation commands

Specifically, we have:

• enable-module
• disable-module
• toggle-module

These can be used to add/remove modules to be loaded or force modules to be
loaded in spite of modules_blacklist. In the typical case, a user is
expected to use them to recover an issue happening due to a module directive
they've added to their loader.conf or because they discover that they've
under-specified what to load.

MFC after: 1 week