Cleanup mark-during-GC debug checks.

Cleanup mark-during-GC debug checks.

* libguile/__scm.h (SCM_DEBUG): add SCM_DEBUG_MARKING_API

* libguile/gc.h (SCM_SET_GC_MARK): depending on
SCM_DEBUG_MARKING_API crash if someone is touching markbits
outside regular hours.

Rename ensure_marking() to scm_i_ensure_marking().

* libguile/inline.h (scm_double_cell, scm_cell): only set mark bits
for debugging if SCM_DEBUG_MARKING_API is unset

* libguile/gc-mark.c: Issue deprecation warning if we are marking
outside of the GC mark phase.