[GSoC] Make vkernels checkpointable - week 2

[Pawel Dziepak]

Hello,

Recently, I have completed support for checkpointing processes with
multiple threads by saving and restoring signal information. That
included keeping information on thread's signal mask and alternate
stack for signal handlers (apparently, signal stack info wasn't saved
even for the main thread). That required changing of the ckpt_siginfo
structure. However, since it is used only by coredump save and
checkpoint restore code there won't be any serious compatibility
consequences.

I've also spent some time on testing to ensure that everything works
well regardless of the hardware architecture (x86, amd64, and
vkernels). Since I haven't found any problems I am now going to (a
week earlier than planned) to start working on reopening network
interface used by vkernel and restoring its configuration.

Paweł