sbang: fixes for sbang relocation

This fixes sbang relocation when using old binary packages, and updates
code in `relocate.py`.

There are really two places where we would want to handle an `sbang`
relocation:

1. Installing an old package that uses `sbang` with shebang lines like
   `#!/bin/bash $spack_prefix/sbang`
2. Installing a *new* package that uses `sbang` with shebang lines like
   `#!/bin/sh $install_tree/sbang`

The second case is actually handled automatically by our text relocation;
we don't need any special relocation logic for new shebangs, as our
relocation logic already changes references to the build-time
`install_tree` to point to the `install_tree` at intall-time.

Case 1 was not properly handled -- we would not take an old binary
package and point its shebangs at the new `sbang` location. This PR fixes
that and updates the code in `relocation.py` with some notes.

There is one more case we don't currently handle: if a binary package is
created from an installation in a short prefix that does *not* need
`sbang` and is installed to a long prefix that *does* need `sbang`, we
won't do anything. We should just patch the file as we would for a normal
install. In some upcoming PR we should probably change *all* `sbang`
relocation logic to be idempotent and to apply to any sort of shebang'd
file. Then we'd only have to worry about which files to `sbang`-ify at
install time and wouldn't need to care about these special cases.

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