Wait for extcap to connect to pipe on Windows.

This prevents dumpcap from failing with timeout exceeded error message
with some extcaps (especially those that display UAC screen) by making
sure that extcap connects to pipe before dumpcap is even started.

Change-Id: I549da9217c7f4ae89509330ca4fa613a119a9523
Reviewed-on: https://code.wireshark.org/review/4428
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>