Pass the number of packets to be captured to "capture()" as an argument,

Pass the number of packets to be captured to "capture()" as an argument,
rather than making it static.

Don't print the "Capturing on <interface>" message until you actually
start capturing, and print it regardless of whether the interface was
explicitly specified or not (that's what snoop and tcpdump do).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@1485 f5534014-38df-0310-8fa8-9805f1628bb7