cpp: Add env var to control editinteractor debug

* lang/cpp/src/editinteractor.cpp (EditInteractor::Private::Private):
Read "GPGMEPP_INTERACTOR_DEBUG" env var.
(EditInteractor::Private::~Private): Close debug file.

While it was possible for the application to control the
debug through setDebugChannel it is often times helpful
to just debug without changing the application using
GPGME