x2gobroker: add maintenance mode feature

Migrated from https://bugs.x2go.org/1318

In case of trouble, this is sometimes needed to block any new incoming connection to the broker (the broker itself, not the x2go server).

For example, lately I did a stupid thing by mistake, I shut down a whole X2Go servers chassis (16 servers at the same time) so 2/3 of my users were kicked off the platform. Problem is that they tried to reconnect before I could restart those servers and thus, the broker did his job and sent them to the remaining servers except that they are 2 times less so they couldn't handle that much people at the same time. We lost precious time by asking people to throttle their connecions and wait for the platform to be ready.

Would be great to have a "maintenance mode" option in the x2gobroker-sessionprfiles.conf (or even globally on the x2gobroker.conf), just a switch with a message you can activate so that, in case of trouble, with this switch enabled, users trying to connect would get a message from the broker saying it is in a maintenance mode and they should try again later.