Calculate and show gaps in summary output

[troglobit]

We could do quite a bit of statistical analysis, but at least increment a counter every time we don't receive the next expected sequence number (counter).

Like this, for example:

root@n2:/usr/src/mcjoin# ./src/mcjoin -j 225.1.2.4 225.4.3.2
joined group 225.1.2.4 on eth0 ...
joined group 225.4.3.2 on eth0 ...
..................^C
Group 225.1.2.4 received 54 packets, gaps: 0
Group 225.4.3.2 received 54 packets, gaps: 30

Received total: 108 packets