Let CLONE_NEWPID properly fail on older kernels.
Let CLONE_NEWPID properly fail on older kernels.

On old kernels where the CLONE_NEWPID flag doesn't even exist, do_fork doesn't
return EINVAL on unknown flags.

dvhh/setuid-sandbox,xuyuu/setuid-sandbox,zeropool/setuid-sandbox,SorinaStoian/setuid-sandbox,fabian-z/setuid-sandbox,airshiplay/setuid-sandbox