PL family references

ASMXREF determines references and their types by analyzing the following PL instructions:

CALL
?LINK
?LOAD
?XCTL
?ATTACH
DCL .. NONLOCAL EXTERNAL (EXTERN)
DCL .. LOCAL EXTERNAL (EXTERN)
EXIT TO (VCON)

External declarations are extracted as EXTERN type references if they are qualified by LOCAL or NONLOCAL attributes. The labels in the EXIT TO instructions are identified as VCON references. All other reference types are classified per the instruction names.