lib: replace conn [write]sockfd members by index

lib: replace conn [write]sockfd members by index

The `connectdata` members `sockfd` and `writesockfd` needed to by either
CURL_SOCKET_BAD or a copy of one of `conn->sock[2]`. When equal to one,
that index was used to send/recv/poll the proper socket or connection
filter chain.

Replace those with `send_idx` and `recv_idx` which are either -1, 0 or 1
to indicate which socket/filter to send/receive on.

Closes #18179