Fix "can't bind to address" error reporting.

Report the actual port used for the listening attempt instead of
server.port.

Originally, Redis would just listen on server.port.
But, with clustering, Redis uses a Cluster Port too,
so we can't say server.port is always where we are listening.

If you tried to launch Redis with a too-high port number (any
port where Port+10000 > 65535), Redis would refuse to start, but
only print an error saying it can't connect to the Redis port.

This patch fixes much confusions.
Cluster: error out quicker if port is unusable

The default cluster control port is 10,000 ports higher than
the base Redis port.  If Redis is started on a too-high port,
Cluster can't start and everything will exit later anyway.
Merge pull request #1555 from mattsta/cluster-port-error-out

Cluster port error out