Provide ability to log client port when logging new connections

[twegener-embertec]

Currently, on new connections, mosquitto logs the IP address and the server port.
However, when the client connections arrive from the local host (e.g. if using a load balancer in front of multiple mosquitto instances), the connections address information is indistinguishable, e.g.:

1606281410: New connection from 127.0.0.1 on port 8884.

It would be useful to be able to log the client port as well, so as to be able to distinguish these connections, and marry them up to log entries in the load balancer. It seems like this would be useful for diagnostics in general, in conjunction with e.g. netstat.
It would also be useful to have the client port logged in the "New bridge connected from" message as well, so as to marry up the client ID to the unique incoming address.

It seems like this should be not too hard.
src/net.c net__socket_get_address could be modified to also extract the client port from the getpeername result, and store that in a second buffer, which context__init could then put into a client_port field in the mosquitto context struct.
Then that could (optionally?) be logged upon new connections.

[ralight]

This will be part of 2.0.