[GSoC] Making vkernels checkpointable - week 5

Hello,

This week I was working on SIGCKPT and SIGCKPTEXIT signal handlers and
network interface restore for 32-bit vkernels. It was rather a trivial
task since there weren't any important differences between vkernel64
and vkernel that I had to deal with.

I also, as Samuel suggested, spent some time on the checkpointing
code. I moved per thread signal information to elf note NT_PRSTATUS
and per process information to NT_PRPSINFO. NT_PRPSINFO also contains
additional field with the number of threads so that we don't have to
compute that number using the size of notes section. Finally, restored
threads are given exactly the same thread IDs as the had at the
checkpointing.

Paweł