sl/symjoin: simplify the implementation of handleUnknownValues()
sl/symjoin: mapAsymTarget() now updates value mapping
sl/symjoin: make checkObjectMapping() less strict

... in order to allow delayed join of sub-SMGs
sl/symjoin: install the hook for delayed join of sub-SMGs
sl/symjoin: update level of prototypes under 0..1 objects

... on the fly and unimplement the updateMayExistLevels() function.
sl/symjoin: fix a wrong use of the return convention

... in cloneSpecialValue(), which caused accessing an uninitialized
value in clonePendingValues() and intermittently prevented the analysis
of test-0195 from a successful termination.