simplify recipes; use remote_file for rootfs
fmt
add debugging support to command runner

Shows command being executed and tees stdout/stderr to os.Stdout/os.Stderr.

Can't use cmd.StdoutPipe or os.StderrPipe as they seem to close the pipe too
early. Using manual os.Pipe and closing instead; there aren't many places
where we have to do this.

Usage: garden -debug