DLT User: add ability to export PDUs

It can be really useful to share hand crafted captures without the need to configure the DLT User table

Change-Id: Iefab00b741f95b1f3a3247cb28ed01a367fa4ff4
Reviewed-on: https://code.wireshark.org/review/4469
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>