JSA::Files: update looks_like_drfile for SCUBA-2

Makemap produces output files (which then appear in the provenance)
which previously passed looks_like_drfile but failed dissect_drfile
(causing all products to be rejected in the provenance processing
stage).
JSA::Files: use six digits for JSA tile numbers

As per previous decision by MJC, JSA tile numbers are six digits
in length.