Pre-emptively fix some C++ warnings.

Moved sepdvdlformat from force.h into a source file to avoid warnings
about an unused variable.  These appeared as soon as a C++ source file
included force.h.  As an additional benefit, makes printing this
information more typesafe; this is how #1294 was found.

Change-Id: Id856fbe193d9b4a74ecfb49cba9eab03bad0a7ba