freebsd: Rewrite netload implementation with getifaddrs and ioctl

Instead of using a documented and official API to get information related
to a network interface, the old implementation uses kvm library to access
kernel memory directly, which is not easy to understand and can be easily
broken by changing kernel symbols or structs. Besides, FreeBSD 11 hides
severai required kernel struct definitions from userspace programs, which
causes compilation error because the definition of 'struct ifaddr' is not
available unless _KERNEL is defined.

This patch is primarily authored by Gleb Smirnoff <glebius@FreeBSD.org>,
with other simple modification made by Antoine Brodin <antoine@FreeBSD.org>,
Koop Mast <kwm@FreeBSD.org> and Ting-Wei Lan <lantw@src.gnome.org> applied.
It was first submitted to FreeBSD ports and has been used by FreeBSD users
for more than 18 months on all supported versions of FreeBSD.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194187
https://svnweb.freebsd.org/changeset/ports/371082
https://github.com/freebsd/freebsd-ports-gnome/commit/be884cd
https://github.com/freebsd/freebsd-ports-gnome/commit/422a418
https://github.com/freebsd/freebsd-ports-gnome/commit/a4b1913
https://bugzilla.gnome.org/show_bug.cgi?id=770165