Make luapower more self-contained

[Cosmin Apreutesei]

One feature of luapower is that it's self-contained, i.e. independent of what's on the host outside the luapower directory. But there are a few loopholes left there for convenience:

• the presence of current directory in LUA_PATH and LUA_CPATH (not in bundled executables)
• obeying LUA_INIT
• looking for shared libraries in system paths (although only as a fallback)

We should probably close these off as they have the potential to interfere with luapower apps and prevent them from starting.