Allow variable functions (variable_del, variable_get, variable_set) to receive an array of name/value pairs

Summary

Allow the variable functions - variable_del, variable_get and variable_set to receive and return name/value pairs allowing multiple variables to be set, deleted or got in a single call.

Background

It is a common design practise for contributed modules to set a number of variables in their install or enable hooks eg:

/**
 * Implements hook_install
 */
function my_module_install() {
  variable_set('my_module_variable', TRUE);
  variable_set('my_module_other_variable', FALSE);
}

Similarly in the uninstall functions, it is common for modules to delete a lot of variables

/**
 * Implements hook_uninstall
 */
function my_module_uninstall() {
  variable_del('my_module_variable');
  variable_del('my_module_other_variable');
}

Likewise, Drush has provided support for extracting variables as keyed arrays -eg:

drush vget fivestar --pipe

Which would return a php array of all variables starting with fivestar.
This has become a useful tool for keeping multiple deployments (development, testing, production) of a Drupal installation in sync without the need to pass around the database.
Examples uses of the variable functions with this patch

Variable set

$my_variables = array(
  'my_module_variable' => 2,
  'my_module_other_variable' => 3
);
variable_set($my_variables);

Variable Get

$defaults = array(
  'my_module_variable' => 'default 1',
  'my_module_other_variable' => 'default 2'
);
$my_variables = variable_get($defaults);
$form['my_module_variable'] = array(
  '#type' => 'textarea',
  '#default_value' => $my_variables['my_module_variable']
);

Variable delete

$my_variables = array('my_module_variable','my_module_other_variable');
variable_del($my_variables);

The patch also includes new tests and update system_settings_form_submit to use the array format of variable_set.