Load libpcap dynamically

This changes how libpcap is loaded. Instead of statically linking
during compile time, it will now be loaded dynamically during runtime.

The only advantage of this is that getting "libpcap-dev" is no longer
a dependency. All you need is to get a C compiler, but otherwise, the
project is dependency free -- in order to build it.

To run it, you still need libpcap.so installed, but this is the default for
all major Linux distros, OpenBSD, FreeBSD, and macOS.