example.c: it is now possible to specify server ip/port.

This makes possible to use the example with IPv6 addresses and/or with a
different Redis instance than 127.0.0.1:6379.
Minimal IPv6 support.

redisContextConnectTcp() is now able to use IPv6 addresses if there is
no IPv4 address found resolving the specified hostname.
Merge branch 'ipv6'