Byte-swap the T and L in TLVs as necessary when reading an NFLOG file.

That means that, when reading a LINKTYPE_NFLOG file, the type and length
values are in the byte order of the host *reading* the file, rather than
the host that *wrote* the file, just as they're in the byte order of the
host capturing the traffic if you're doing a live capture of NFLOG
messages.

That way, when reading a LINKTYPE_NFLOG file and writing another one
from those packets, the type and length in the output file will be in
the byte order of the host writing the file, rather than the byte order
of the host that wrote the input file.

Export the nflog.h file containing the declarations and definitions we
need, for use by tcpdump and other programs reading LINKTYPE_NFLOG
files.

Put the bulk of the byte-swapping code into a common routine, for use by
pcap and pcap-ng readers, while we're at it.