stdlib: readback fn captures stdout but still echoes out stderr from the shell call to our program's stderr

[rsaarelm]

Should probably redirect subcommand's stderr to /dev/null or something.

[FeepingCreature]

This is intentional. Mostly when I call out to other programs (ie. gcc), I want to echo stderr to give the user an impression when something goes wrong. Belongs with #59 imo: having a configurable process class lets us just close stderr.