The remaining issue in `win-askappend-multi' appears to have been

The remaining issue in `win-askappend-multi' appears to have been
caused by the MessageBox() internal message loop eating WinSock
FD_READ notifications, which then don't reappear afterwards because
you have to explicitly prod a socket in order to get a repeat
notification on it.

Hence, here's a piece of infrastructure which seems to sort it out:
a new winnet.c function called socket_reselect_all(), whose function
is to go through all currently active sockets and re-run
WSAAsyncSelect() on them, causing repeat notifications for anything
we might have missed. I call this after every call to MessageBox(),
and that seems to solve the problem.

(The problem was actually masked in very recent revisions, probably
by the reinstatement of pending_netevent in r7071. However, I don't
believe that was a complete fix. This should be.)

[originally from svn r7077]
[r7071 == 57a763b0ec55b84d6a515e6d860759898d648869]