multi_socket: re-use of same socket without notifying app

When a hostname resolves to multiple IP addresses and the first one
tried doesn't work, the socket for the second attempt may get dropped on
the floor, causing the request to eventually time out. The issue is that
when using kqueue (as on mac and bsd platforms) instead of select, the
kernel removes the first fd from kqueue when it is closed (in trynextip,
connect.c:503). Trynextip() then goes on to open a new socket, which
gets assigned the same number as the one it just closed. Later in
multi.c, socket_cb is not called because the fd is already in
multi->sockhash, so the new socket is never added to kqueue.

The correct fix is to ensure that socket_cb is called to remove the fd
when trynextip() closes the socket, and again to re-add it after
singleipsocket(). I'm not sure how to cleanly do that, but the attached
patch works around the problem in an admittedly kludgy way by delaying
the close to ensure that the newly-opened socket gets a different fd.

Daniel's added comment: I didn't spot a way to easily do a nicer fix so
I've proceeded with Ben's patch.

Bug: http://curl.haxx.se/bug/view.cgi?id=3017819
Patch by: Ben Darnell