ProxySQL should fail immediately if it cannot bind to admin port

[ovaistariq]

Currently ProxySQL starts even if its unable to bind to the admin port.
ProxySQL should fail if it unable to bind to the admin port.

Otherwise there is no way for a lay-man to detect why a client connection to the admin port hangs until it times out. I faced this issue today during a race condition between restarting ProxySQL and invoking Percona's ProxySQL scheduler.

[renecannao]

I am not sure if failing immediately is the right approach, considering that it could bind to a multitude of TCP ports and unix socket domains.