transfer: adjust_pollset improvements

transfer: adjust_pollset improvements

- let `multi_getsock()` initialize the pollset in what the
  transfer state requires in regards to SEND/RECV
- change connection filters `adjust_pollset()` implementation
  to react on the presence of POLLIN/-OUT in the pollset and
  no longer check CURL_WANT_SEND/CURL_WANT_RECV
- cf-socket will no longer add POLLIN on its own
- http2 and http/3 filters will only do adjustments if the
  passed pollset wants to POLLIN/OUT for the transfer on
  the socket. This is similar to the HTTP/2 proxy filter
  and works in stacked filters.

Closes #12640