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