Allow tracing kernel threads

[Xyene]

Currently, tracing them fails because we can't open /proc/$pid/exe, but we can figure out if we're looking at a kernel thread (its parent's PID should be 2), and use vmlinux instead.

[hlian]

@Lilydemi is going to take this issue!

[ameya-deshmukh]

@Xyene is this open to take up? Would love to give it a go.

[cgaebel]

It's open.

[ameya-deshmukh]

Thanks!