RFR: Port of FreeBSD *context routines to DFly

[Matt Emmerton]

I have completed the port of [set|get|make|swap]context from FreeBSD to
DFly.

The latest patch set against HEAD (current as of Jan 12, 00:00 EST) is
available here: http://www.gsicomp.on.ca/~matt/dfly

I (and others) would really like to see this get into the release, so I
would appreciate reviews and comments.  Nothing too earth-shattering here -
it's a straight port with some minor modifications due ot Matt's recent
VKERNEL and trapframe changes.

Regards,
--
Matt Emmerton