Fix a number of SCM/int confusions.

Fix a number of SCM/int confusions.


Contains commits:

Stop SCM/int confusion in stencil head registration


Stop SCM/int confusion in program-option code


Stop SCM/int confusion in Drum_notes_engraver


Stop SCM/int confusion for grob interface table


Stop SCM/int confusion in function documentation tables


Stop SCM/int confusion for property profiling


Use Protected_scm.is_bound () with Scheme engravers


Add Protected::is_bound () function

Since SCM_UNBNDP has problems with arguments of type Protected_scm, an
alternative member function seems like a viable alternative.