libifconfig: Add netlink based link and address helper functions

[saheed]

Summary

Adds the following RTM_*LINK related functions:

• ifconfig_if_up and ifconfig_if_down to bring an interface up or down
• ifconfig_get_mac and ifconfig_set_mac to retrieve and set MAC address

ifconfig_set_mac depends on https://reviews.freebsd.org/D51922

and the following RTM_*ADDR related functions:

• ifconfig_add_inet and ifconfig_del_inet to add and remove IPv4 addresses
• ifconfig_add_inet6 and ifconfig_del_inet6 to add and remove IPv6 addresses

The RTM_*ADDR related functions are derived from ifconfig

[obiwac]

overall looks good. will look at this deeper and do a proper review later - I need to get the netlink snl stuff back into my mental context window :)

lib/libifconfig/libifconfig_nl.c

1–27

Someone can correct me if I'm wrong but I believe it's preferred not to include the whole license text in new files (i.e. just keep SPDX tag and copyright line)

[saheed]

Since the changes are quite large, I'll split them into smaller revisions :)

lib/libifconfig/libifconfig_nl.c

1–27

You're right. I'll replace it with just SPDX tag and copyright line

[saheed]

This revision has been split among:

• https://reviews.freebsd.org/D52130
• https://reviews.freebsd.org/D52131
• https://reviews.freebsd.org/D52128