Fix typo in FORMAT_EXTENSIONS which breaks universe.
Add in support to optionally pin the swi threads.

Under enough load, the swi's can actually be preempted and migrated
to other currently free cores.  When doing RSS experiments, this lead
to the per-CPU TCP timers not lining up any more with the RX CPU said
flows were ending up on, leading to increased lock contention.

Since there was a little pushback on flipping them on by default,
I've left the default at "don't pin."

The other less obvious problem here is that the default swi
is also the same as the destination swi for CPU #0.  So if one
pins the swi on CPU #0, there's no default floating swi.

A nice future project would be to create a separate swi for
the "default" floating swi, as well as per-CPU swis that are
(optionally) pinned.

Tested:

* parallel TCP tests (2 x 1g unfortunately for now);
  CPU: Intel(R) Xeon(R) CPU E5-2650

Note:

This is based on some initial investigation into RSS/TCP stack lock
contention on FreeBSD-HEAD whilst at Netflix in January 2014.