Previously, "_start" was run as part of module instantiation, which
meant it was always run, even for wasm modules that weren't being
loaded as commands. Now, just invoke it from the wasmtime driver,
which for now is the only place that runs wasm modules as actual
commands.
Also, stop recognizing the old "main" entry point, which tools have
stopped using a while ago, and switch to start recognizing the ""
entrypoint.
8a1b7965d8