freebsd_network: Rework taskqueues implementation.

* Use FreeBSD's queue management directly, only retaining thread
creation and synchronization functions from Haiku.

* Use FreeBSD's task structure declarations.

* Use a standard non-FAST taskqueue for taskqueue_thread.

* Rewrite header and adjust consumers as appropriate.

freebsd compat: Updates and additions for a newer iflib.

freebsd_iflib: Merge changes from FreeBSD's master branch.

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.