avoid zombies
avoid zombies

When processes created by `driver_exec` terminate, they are not reaped because dnscat the doesn't `waitpid()`.
This change portably sets the `SIGCHLD` handler to `SIG_IGN` which causes zombie processes to be reaped automatically.
EricSB/dnscat2,iagox86/dnscat2,iagox86/dnscat2,EricSB/dnscat2,iagox86/dnscat2,iagox86/dnscat2,EricSB/dnscat2,iagox86/dnscat2