feat(protocols): the default engine has no LINKTYPE_NULL (BSD loopback) dissector #1574

The default engine has no dissector for LINKTYPE_NULL (0, BSD loopback), so frames on such an interface are never dissected. scapy dissects them.

Repro: on main (a3da376dd), frames 3 and 4 of examples/captures/many_interfaces.pcapng sit on interface 10, whose link type is LINKTYPE_NULL. Their protocol chain is just NULL, while the frames on interface 0 read Ethernet:IPv4:UDP:Raw.

• LinkType.NULL is missing from PCAPNG.__proto__, whose keys are only 1, 228 and 229.
• pcapkit/protocols/link/ has no loopback protocol.

Cause of a misfiled gap: #1502 blamed the missing datagrams in many_interfaces.pcapng (34 against scapy's 36) on LINKTYPE_RAW. The #1573 worker found that this capture has no type-101 frames; the gap is these two NULL frames. So the many_interfaces labels in the Gap(1502) row belong to this issue.

Expected: a BSD loopback link layer. It is a 4-octet address-family header in the capturing host's byte order, with 2 = IPv4 and 24/28/30 = IPv6. It dispatches to IPv4/IPv6 and is registered for LINKTYPE_NULL in the PCAP frame and PCAP-NG dispatch tables, so the many_interfaces datagrams match scapy's.

Blocked: this waits until #1550, #1563 and #1573 merge. It edits the same dispatch tables (protocols/misc/pcap/frame.py, protocols/misc/pcapng.py) and Gap rows as #1502, and it will land together with #1502.