Zero-initialize registers

[jpsamaroo]

This ensures that return values of simple programs are predictable, and makes debugging a whole lot easier.

[coveralls]

Coverage increased (+0.005%) to 96.054% when pulling 6de5e56 on jpsamaroo:zero-init-reg into 089f627 on iovisor:master.

[qmonnet]

Note that unless I'm mistaken, this may widen the gap between uBPF and kernel eBPF if programs start relying on 0-initialised registers (In the kernel, the verifier ensures that no register is used uninitialised). It may also introduce a divergence within uBPF between the VM and the JIT-compiler.

[jpsamaroo]

It's also my intention to add support for verification similar to what the kernel does, which would give us the safety properties. But since we aren't currently checking for use of uninitialized registers, I don't see how this patch is in itself harmful.

I'll have to look into the JIT and see if I can add this zeroing behavior there as well. If not, then I can understand why this wouldn't be merge-worthy.

[jpsamaroo]

I think a verification pass is more than sufficient to check for use of uninitialized registers, so this PR is not needed.