sbang: vendor sbang

`sbang` now lives at https://github.com/spack/sbang, and it has its own
test suite that's more extensive than what's in Spack. We'll leave sbang
tests to sbang from now on, and just vendor `bin/sbang` directly.
Remaining `sbang` tests have to do with patching files, not with
`sbang`'s functionality.

This update also fixes a bug with `sbang` and multiple command line
arguments that was introduced in #19529. See:
  * https://github.com/spack/sbang/pull/1
  * https://github.com/spack/sbang/pull/2

- [x] include latest `sbang` from https://github.com/spack/sbang
- [x] remove old `sbang` tests from Spack
- [x] update `COPYRIGHT` and `cmd/license.py`

COPYRIGHT

@@ -28,9 +28,11 @@ text in the license header:
 
 External Packages
 -------------------
-Spack bundles its external dependencies in lib/spack/external.  These
-packages are covered by various permissive licenses.  A summary listing
-follows.  See the license included with each package for full details.
+
+Spack bundles most external dependencies in lib/spack/external. It also
+includes the sbang tool directly in bin/sbang. These packages are covered
+by various permissive licenses. A summary listing follows. See the
+license included with each package for full details.
 
 PackageName: argparse
 PackageHomePage: https://pypi.python.org/pypi/argparse
@@ -76,6 +78,10 @@ PackageName: ruamel.yaml
 PackageHomePage: https://yaml.readthedocs.io/
 PackageLicenseDeclared: MIT
 
+PackageName: sbang
+PackageHomePage: https://github.com/spack/sbang
+PackageLicenseDeclared: Apache-2.0 OR MIT
+
 PackageName: six
 PackageHomePage: https://pypi.python.org/pypi/six
 PackageLicenseDeclared: MIT