Provide `basename` for PPSUB.REFERENCE and PPSUB.INPUT

[Michael Wood-Vasey]

Provide basename for PPSUB.REFERENCE and PPSUB.INPUT since they are truncated to 80 characters total line width and so the most important details are lost. The basename should be descriptive enough and will be far more useful than the directory information. Alternatively, as long was we're using HIERARCH keywords, do all HIERARCH-aware libraries understand line continuations as well?