Add support for DSA link-layer types

Add support for DSA link-layer types

Linux kernel 4.20 and greater can report what type of Distributed Switch
Architecture tagging protocol is used on the DSA master/management
interface. We need to map the protocol to a specific DLT and linktype
value in the pcap file because these protocols typically cannot be
decoded simply by making use of heuristics. The sysfs attribute that is
being checked and parsed is documented in this commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a3d7e01da06013dc580641a1da57c3b482d58157

For now, the description of the Broadcom 4 byte Ethernet switch tagging
protocol is provided and more tags can be added in the future using the
same infrastructure.