genl: add new command to list genetlink(4)

This commands list genetlink protocols and its operations and
capabilities

Name: nlctrl
ID: 0x10, Version: 00, header size: 2, max attributes: 10
supported operations:
- ID: 0x3, Capabilities: 0xe (can modify; can get/dump;
has policy)
multicast groups:
- ID: 0x30, Name: notify
Name: carp
ID: 0x11, Version: 00, header size: 2, max attributes: 2
supported operations:
- ID: 0x1, Capabilities: 0xe (can modify; can get/dump;
has policy)
- ID: 0x2, Capabilities: 0xb (requires admin permission;
can modify; has policy)

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D40330