cfilters: fix busy loop on blocked transfers

cfilters: fix busy loop on blocked transfers

When a transfer gets paused after the connection has been established,
any data sitting in the kernel socket buffers will no longer get read.

Prevent the sockets form being added to the pollsets, because they
will trigger POLLIN endlessly and cause a busy poll loop. Same in event
based processing.

Reported-by: Harry Sintonen
Fixes https://github.com/curl/curl/issues/21671
Closes https://github.com/curl/curl/pull/21675