Add getter for local port of server socket

Having a getter for the local port of the server socket permits the
use of port 0.

While creating a ServerSocket, port 0 means the socket will use any
available port of the system. To know which port was chosen, you need
the getter added by this commit.
Merge pull request #42 from s-bernard/get_syslog_port

Add getter for local port of server socket