Add a Buffer to wtap_pkthdr to hold file-type-specific packet metadata.

For example, this can be used for pcap-ng options not mapped to
file-type-independent metadata values.

Change-Id: I398b324c62c1cc1cc61eb5e9631de00481b4aadc
Reviewed-on: https://code.wireshark.org/review/5549
Reviewed-by: Guy Harris <guy@alum.mit.edu>