This does several things:
- Add `sbang`: a script to run scripts with long shebang lines.
- Documentation for `sbang` is in `bin/sbang`.
- Add an `sbang` hook that filters the `bin` directory after install
and modifies any scripts wtih shebangs that are too long to use
`sbang` instead.
- `sbang` is at the top level, so it should be runnable (not much we
can do if spack itself is too deep for shebang)
- `sbang`, when used as the interpreter, runs the *second* shebang
line it finds in a script.
- shoud fix issues with too long shebang paths.
