checkpoint/restart milestone 1

[Kip Macy <kmacy@xxxxxxxxxxx>]

At his talk at BSDCon, Davis Brooks mentioned the need for a checkpoint/
restart mechanism. After talking briefly about it with Dave and Matt I
thought it would make an interesting weekend project. My initial
estimate of a weekend for barebones support proved correct. Right now
it only restores userland state (files aren't reopened, and signal
dispositions are not restored). Adding the restoration of kernel state 
shouldn't take long. This was done under 4.8 (I don't have a box running
DragonFly at the moment), but should port fairly trivially to DragonFly.

In order to be general-purpose it really needs to also support rfork
based threads. However, before I do that I want to clean up their
signalling semantics to bear some resemblance to POSIX, so that will
have to wait until next week at the earliest.

Unless someone has a compelling need for it, I'm going to wait until
signal dispositions are being restored to submit. I just wanted to let
it be known that it is in the pipe.

						-Kip