espintcp: recv() should return 0 when the peer socket is closed
man 2 recv says:
RETURN VALUE
When a stream socket peer has performed an orderly shutdown, the
return value will be 0 (the traditional "end-of-file" return).
Currently, this works for blocking reads, but non-blocking reads will
return -EAGAIN. This patch overwrites that return value when the peer
won't send us any more data.