Create dpctl command for ns3 use.

This int dpctl_exec_ns3_command () is used bu ns3 simulator to execute a dpctl command. This implementation is similar to main. Also, functions dpctl_send_and_print and dpctl_transact_and_print were defined as weak, allowing ns3 to overrid them and reimplement the proper send methods.
Putting static modifiers back into dpctl parsers.