SCUBA2: use a generic name as this recipe is widely useful

Since we already have REDUCE_SCAN_FAINT_POINT_SOURCES, and the
original CLS recipe was an extension of that to use a jack-knife
method to remove residual large-scale structure in maps, and that this
recipe has wider application that just CLS it makes sense to give it a
more generic name, and tailor it to CLS as necessary.
SCUBA2: shortened names for the faint point sources recipes

It's one thing having long recipe names that are automatically
selected, but expecting users to type them in is perhaps a bit
much. Therefore create shortened versions that still describe the
essence of what the recipe is about. The exception is REDUCE_CLS of
course which was previously renamed and for now, it's exactly the same
recipe.
SCUBA2: update internal documentation to reflect name change