freebsd_iflib: Import.

This is the iflib from FreeBSD 12.0 with a few select patches from
FreeBSD trunk applied (and a lot of #ifdef __HAIKU__s, as you can see.)

There are certain things it adds to SYSINIT that can't be un-initialized
(like the grouptaskqueues), and so rather than adding this to every
driver unnecessarily, a separate .a for it makes the most sense.