Backwards compatibility of old shell scripts

[AndrewFasano]

Our busybox shell is instrumented to help us analyze the behavior of shell scripts at runtime. To do this, we swap out an original /bin/sh -> orig busybox with /bin/sh -> our busybox.

This has caused issues in a few ways with backwards compatibility

• A function with no body was previously valid, but is now a syntax error. Fixed by 6106cbd.
• It was previously possible to declare a local variable outside of a function body, this is no longer valid.

There are more issues - we can track them in this issue. Ideally we'd either forward-port support for these "features" or find ways to detect when we can/can't safely use our shims.