SOCK_NOSPACE seems to indicate that the reason we're not sending packets
is a lack of send buffer space.  Effects:

	- In tcp-input.c: when NOSPACE is set, allows autotuning code to
	  increase the send buffer size (in tcp_check_space).
	- Also in tcp-input.c: prevents a decrease of the congestion
	  window in tcp_cwnd_application_limited.

Most places where its set are just before waiting for some kind of
socket memory.