Set the extcap working directory on Windows.

Set the extcap working directory on Windows.

On Windows, prepend the main program directory to %Path% when spawning
extcap processes. This lets us place androiddump in extcap while allowing
it to locate its DLLs.

Change-Id: I406c47ce71323266d5f14fb596931398464e452d
Reviewed-on: https://code.wireshark.org/review/8057
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>