POLL/SELECT ON TCP/UDP SOCKETS NEEDS READ-AHEAD

```
Description: poll()/select() only works for availability of buffered TCP/UDP
read data (when read-ahead is enabled). The way writing is
handled in the network layer, either (1) If CONFIG_UDP/TCP_WRITE_BUFFERS=y
then we never have to wait to send; otherwise, we always have
to wait to send. So it is impossible to notify the caller
when it can send without waiting.

An exception "never having to wait" is the case where we are
out of memory for use in write buffering. In that case, the
blocking send()/sendto() would have to wait for the memory
to become available.
Status: Open, probably will not be fixed.
Priority: Medium... this does effect porting of applications that expect
different behavior from poll()/select()
```