new option --prefix for _GenerateDissectors.pl

So systems where X11 and 3rd party software do not live in /usr (for
example *BSD systems) may use an appropriate installation location
using something like

    make install PREFIX=/usr/local
new option --display for interceptor

So a different ip and port for the interceptor may be specified.
endpoint_cmdline: don't use /proc/net/tcp if it's not there