Create the spill path only once.
Do not spill on send failures, and shift queue only on success.
If we are stopping, don't get stuck on sending.
If failing, try uncorking the socket first.
Make the buffer size minimum and quantum both one megabyte.
Add send buffer size config.
Use the send buffer size.
Rename as open_send_socket_eventually().
Make low-level socket errors just warns, the caller knows if fatal.
Make the socket errors a little more verbose.
Add "random" fuzz to sleeping while trying to open.
Needing abortable send-eventually lead into major refactoring.
Fail gracefully if no queue socket.
Capture the sendto errno so that we have more detail.

Sleep for a while if:
- if traffic jam ("Resource temporarily unavailable")
- if send interrupted by signal
Larger rcvbuf.
setsockopt() any non-zero return is bad.
Implement --rate=N throttling.