MEDIUM: tcp: make tcp_connect_probe() consider ERR/HUP

MEDIUM: tcp: make tcp_connect_probe() consider ERR/HUP

Now that we know what pollers can return ERR/HUP, we can take this
into account to save one syscall: with such a poller, if neither are
reported, then we know the connection succeeded and we don't need to
go with getsockopt() nor connect() to validate this. In addition, for
the remaining cases (select() or suspected errors), we'll always go
through the extra connect() attempt and enumerate possible "in progress",
"connected" or "failed" status codes and take action solely based on
this.

This results in one saved syscall on modern pollers, only a second
connect() still being used on select() and the server's address never
being needed anymore.

Note that we cannot safely replace connect() with getsockopt() as the
latter clears the error on the socket without saving it, and health
checks rely on it for their reporting. This would be OK if the error
was saved in the connection itself.