GSoC: Make vkernels checkpointable

[Irina Preșa]

Hello everyone!

My name is Irina Presa and this summer I'll be working on making
vkernels checkpointable as a GSoC project.

Saving the vkernel's state will allow us to migrate a complete
DragonFly operating system running on the vkernel platform.
Here is a brief summary of what I'll be doing:

First, in order to be able to checkpoint multithreaded applications
(such as vkernel), I'll have to add support for TLS save and restore.

After that I will go on implementing the actual vkernel checkpointing system.
This will imply:
    - dumping the vmspaces and recreating them on restore.
    - upon SIGCKPT signal, save network drivers and terminal states
along with other information needed for a clean restore.


Regards,
Irina