registrar: add filesetpredicate to mark a function as fileset predicate filesetpredicate is used to replace fileset.predicate in subsequent patch. This patch also adds loadpredicate() to fileset, because this combination helps to figure out how the name of "status caller" (or "existing caller") predicate is put into _statuscallers (or _existingcallers). Listing up loadpredicate() in dispatch.extraloaders causes implicit loading fileset predicate functions at loading (3rd party) extension.