Make stdin/stdout/stderr be 3/4/5, and alias them to 0/1/2.

This allows us to unambiguously detect when code is doing e.g.
write(1) rather than write(STDOUT_FILENO) and transparently do
the right thing.
Add DISABLE_UNSAFE_OPTS setting.

This allows the compilation to explicitly opt-out of LLVM optimisations
that are suspected to be unsafe.