Use old-style connects if new-style connects fail

For reasons we don't understand the new-style connects fail on Windows
if used from the resolver application. Debugging shows that the function
pointers of the signals are different from the ones Qt expects.

(cherry picked from commit d44b78f1b988dbc9dbe80453a6a39b9cd48eb685)

• GnuPG-bug-id: T6813