net_Read: rewrite, fix corner cases (fix #8972)

net_Read: rewrite, fix corner cases (fix #8972)

This new implementation opportunistically tries to read data before
invoking poll(). This reduces latency when data is already waiting in
buffers, allows receiving pending data in non-blocking fashion even if
the object has been killed. This also gives GnuTLS a chance to dequeue
data from internal buffers.

Also the corner case of 0 read should now be handled correctly.

The corner case of an error after success read is no longer handled.
This was race-prone and useless.