luahttp: fix scope of url handler variables

luahttp: fix scope of url handler variables

The url handlers were declared with local scope, meaning local to the
lua script file I assume. Since the interface doesn't wait at the end
of the lua script anymore, they would fall out of scope after interface
initialization. Then, when the interface was first accessed, garbage
collection was somehow triggered, unregistering all url callbacks,
incidentally triggering a deadlock.

Fixes #6673