client to retry to connect until it succeeds

[Antoine Martin]

When we know where we want to connect, we may want to retry a number of times until we succeed.
This would allow the server and client to be started in parallel.
ie: ​https://github.com/mviereck/x11docker/issues/167#issuecomment-506864026

There should probable a configurable limit of retries.

[Antoine Martin]

We now retry up to XPRA_CONNECT_TIMEOUT (defaults to 20 seconds):

• r23059 for unix domain sockets: retrying every 100ms
• r23060 for INET sockets: retrying every second

Not doing VSOCK or named-pipes.

[Antoine Martin]

Improvements:

• r23078 better socket lookup
• r23077 also wait for servers if the socket does not exist at all