Fix #104, #54: issues with overlong shebang in deep directories.

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.

lib/spack/llnl/util/filesystem.py

@@ -25,7 +25,8 @@
 __all__ = ['set_install_permissions', 'install', 'install_tree', 'traverse_tree',
            'expand_user', 'working_dir', 'touch', 'touchp', 'mkdirp',
            'force_remove', 'join_path', 'ancestor', 'can_access', 'filter_file',
-           'FileFilter', 'change_sed_delimiter', 'is_exe', 'force_symlink']
+           'FileFilter', 'change_sed_delimiter', 'is_exe', 'force_symlink',
+           'copy_mode', 'unset_executable_mode']
 
 import os
 import sys
@@ -158,6 +159,14 @@ def copy_mode(src, dest):
     os.chmod(dest, dest_mode)
 
 
+def unset_executable_mode(path):
+    mode = os.stat(path).st_mode
+    mode &= ~stat.S_IXUSR
+    mode &= ~stat.S_IXGRP
+    mode &= ~stat.S_IXOTH
+    os.chmod(path, mode)
+
+
 def install(src, dest):
     """Manually install a file to a particular location."""
     tty.debug("Installing %s to %s" % (src, dest))