freebsd_iflib: Remove Haiku-specific semaphores code and use msleep instead.

Now that the interlocking issues are resolved with the previous commit,
using the main TQ_SLEEP mechanism no longer causes deadlocks and
network I/O freezes, so we can drop the custom code and use FreeBSD's
code instead.

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.