Support exec mode

[rossica]

"Exec mode" behaves exactly as -e does in netcat, in that is spawns a subprocess and uses the stdin and stdout pipes to read/write traffic to the network.
On the server, each new connection (if keep-open is used) would spawn a new subprocess, and when the process exits the connection closes, or the connection closes, the subprocess exits.

Need to either write my own OS-abstraction layer here, or use a library, since there's no STL way to do this, it seems.