Optionally limit packet count

[pchaseh]

Adds a new flag, -c, which allows for the capture to stop when a certain number of matched packets have been hit similar to what tcpdump offers. Having the capture utility be capable of exiting on this condition is helpful in the following cases:

• The application is ran with a filter that may match a significant number of traffic, but the user only wants a fixed size sample
• One wants to run the program unattended until a certain amount of packets have been captured, but without the burden of having to kill it after a certain amount of time using timeout and risking either ending up with too few or too many packets

These changes can be tested by running xdpcap with the -c flag set to a non-zero value, observing it exiting, and then verifying that the resulting packet capture only has the requested amount.