D7 port by jonhattan in #795574: Port Maintenance Helper to D7
Adds a new "restricted login mode" variable and the permission to login is now renamed and only required to login when that mode is set.
This makes it easier to put a site into the restricted mode and configure which roles can login in those time periods.
Development release for the 6.x branch of Drupal. Should be generally functional, but may not work perfectly.