findmissing: getopen: add debug option

Add debug option (-d, --debug) to enable debug logs.

Note: Python's logging default writes logs to stderr.  Thus, the patch
doesn't break such `./getopen.py -c -d | tee -a conflicts` usage if any.

BUG=none
TEST=./getopen.py -c -d

Change-Id: I086d897adb69d22fc09e01694a3d52ca923c5d82
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3463399
Tested-by: Tzung-Bi Shih <tzungbi@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Commit-Queue: Tzung-Bi Shih <tzungbi@chromium.org>