Add expand=False option for URL downloads.

- Allows skipping the expand step for downloads.
  - Fixed stage so that it knows expansion didn't fail when there is a
    no-expand URLFetchStrategy.

- Updated docs to reflect new option, and provided an example.

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', 'remove_dead_links', 'remove_linked_tree']
+           'FileFilter', 'change_sed_delimiter', 'is_exe', 'force_symlink',
+           'set_executable', 'remove_dead_links', 'remove_linked_tree']
 
 import os
 import sys
@@ -345,6 +346,12 @@ def traverse_tree(source_root, dest_root, rel_path='', **kwargs):
     if order == 'post':
         yield (source_path, dest_path)
 
+
+def set_executable(path):
+    st = os.stat(path)
+    os.chmod(path, st.st_mode | stat.S_IEXEC)
+
+
 def remove_dead_links(root):
     """
     Removes any dead link that is present in root