failed upgrades should preserve sockets

[Antoine Martin]

We currently delete the previous server sockets and if the upgrade fails then we are left with no sockets..

Not easy to solve.
Options:

• pass sockets from the old server to the new one with ​sendfd - would require connecting to the old server using unix-domain-sockets, which may not be possible or require authentication, but this would allow us to preserve all the different sockets
• delay creating sockets until we have taken over the display - but if we fail to create them, then we will have to exit and leave the vfb running?
• temporarily move them out of the way, and move them back if we fail - easier?

Other issues:

• backwards and forwards compatibility
• request clients to re-connect? (to the same address only?)