`sbang` was previously a bash script but did not need to be. This
converts it to a plain old POSIX shell script and adds some options. This
also allows us to simplify sbang shebangs to `#!/bin/sh /path/to/sbang`
instead of `#!/bin/bash /path/to/sbang`.
The new script passes shellcheck (with a few exceptions noted in the file)
- [x] `SBANG_DEBUG` env var enables printing what *would* be executed
- [x] `sbang` checks whether it has been passed an option and fails gracefully
- [x] `sbang` will now fail if it can't find a second shebang line, or if
the second line happens to be sbang (avoid infinite loops)
- [x] add more rigorous tests for `sbang` behavior using `SBANG_DEBUG`
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