Fix regression from #14623 - use debug SVC handler only on interactive sessions ...

Fix regression from #14623 - use debug SVC handler only on interactive sessions (#15210)

Unfortunately #14623 changed from the deprecated IsInteractiveSession to
IsWindowsService without recognising that they are the complement of
each other.

This means that Windows SVC control is not working correctly. This PR
adds some Tracing statements but also fixes the bug.

Fix #15159

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>