Uses libpcap and hands each packet to the parser.

Grammars often depend on the knowledge of the length of the content.
This is especially the case for Ethernet packets. Thus, the modified
pac-driver needs to provide this information to Hilti. The caplen
provided by libpcap as uint32_t is added at the very beginning to the
input of Hilti. In a grammar, this is the first data to be interpreted.
By default, a grammar expects big endian; thus, caplen is converted
accordingly.

Chosen such that it should only take up one line,
this keeps the output clearer.

Add a new command line option -f. The file is read using libpcap. This
shares a lot of code with listening on a network interface.

The grammar networkinterface.pac2 was renamed to libpcap.pac2 because it
is now used for both reading from pcap files and network devices.

There is now an ambiguity with parsers/libpcap.pac2 and
parsers/pcap.pac2, there should be a way to resolve this.

When using libpcap to read from a network interface or a pcap file,
hand over the packet capture time and also the total length of the
packet; not only the captured length.

Currently, the timestamps are handled as being 64 bit. This has to be
generalized to cover platforms where time_t and suseconds_t are 32 bit,
as well.