add maintenance mode
module to show system down message
config cli will also work with Friendica without change - so bringing it over
Merge https://github.com/friendica/friendica into pull
run check_config() in a separate process
Merge pull request #585 from fermionic/20130114-maintenance-mode

add maintenance mode
check plugins in separate function
remove commented-out check_config()
Merge pull request #586 from fermionic/20130114-maintenance-mode

check plugins in separate function