bpftrace and user program both write to proctrace stdin

[zmitchell]

proctrace reads bpftrace output, but output from the user program gets mixed in. The output doesn't parse properly, so it's not an issue of data correctness, but you end up seeing a lot of "failed to parse" lines. We could fix this from two angles:

• Redirect the user program output somewhere else
• Open a different set of pipes for bpftrace