Add nsenter command

iolimit: Print current limits if no settings

Finish "iolimit" command

iostat, pressure: QEMU support

Cleanup qemu-related code to cgroup/fs.go, and add qemu support for iostat

pressure: Cleanup code

Add killMode to killall command

killall: print any errors

findpid: Explicitly show <none> if PID is not in a container

Add a --version flag for the root command

Add findpid subcommand