core: extend help/usage output

This patch increase the output of usage()/help() by providing a short
description of each module:

Usage: captcp [-h | --help] [--version] <modulename> [<module-options>] <pcap-file>
Available modules:
   stacktrace      - Hook into Linux Kernel to trace cwnd, ssthresh, ...
   show            - Tcpdump/tshark like mode
   spacing         - Time between packets and acks
   statistic       - Show statistic about all connections
   payloadtimeport - Show distribution of data over ports
   timesequence    - Plot a Time-Sequence graph
   geoip           - Show country/location information about peers
   inflight        - Visualize all packets in flight and not ACKed
   sound           - Play sound based on payload/ack packets
   connection      - Visualize all communicating peers
   throughput      - Graph the throughput over time graph
   template        - Metamodule to generate template files for Gnuplot
   sequencegraph   - Graph a TCP sequencegraph (flow graph)

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>