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