Implement SSVN default values

Implement SSVN default values. Need to be able to define SSVN references with a corresponding default value, as KILLing certain MWAPI nodes in ^$WINDOW is supposed to restore the default attribute instead of making the node undefined. KILL will need a way to trigger this, as well as KVALUE. May be best handled in symtab(), as thus far, only process-private SSVNs have this requirement.