a simple hack to make caller apparent

a simple hack to make caller apparent

Teliva isn't yet smart enough to know the caller of an indirect function
where the function being called goes through a local variable.

I'd expected fixing this to be a long death march. However, there's a
shockingly easy fix: just make every indirect call go through an
additional direct function call.

My policy for zet.tlv was that function 'main' could open any file. This
stopped working since I introduced spawn_main. But with this commit it's
working again.

I can also drop all my special-casing of 'main' since it's now a regular
Lua call.

We still can't rely on the caller of an indirect call. That affects
start_reading and start_writing, which really need to be part of the
framework.