Always use non-blocking sockets

This is a larger, and a long needed cleanup, inspired by MDEV-39473 (client failing on macOS)

• Always use non-blocking sockets. There is no reason to use blocking sockets, like, at all, because of the item below, and because async API fully depends on non-blocking already.
  The blocking in poll over non-blocking sockets is already in place, and correct timeout handling.
• Remove (sometimes painful) timeout handling via setsockopt - it can be a problem on OSes that are rarely tested, e.g macOS. The problem is already solved for non-blocking sockets via poll/select
• Make OpenSSL and GNUTLS use underlying pvio IO functions via BIO . It would handle timeouts correctly, and also the async API.
  potentially it could make OpenSSL via named pipe possible, there is already in plans for the server.
• Remove now-obsolete pvio_blocking etc, constant switchery from blocking modes, setsockopt, Linuxisms like MSG_DONTWAIT etc.