Implement an option -o ("Once only") that allows this netcat utility
to terminate at the first EOF condition on local input (as opposed to
waiting for the network end to shut down). Can be useful when talking
to network servers that never drop the connection on their end, like
an port 9100 printer server in my case.
I've also submitted that to the OpenBSD maintainer, so in case they
accept, we can later on undo my local patch.
Approved by: dd (maintainer)