registrar: add filesetpredicate to mark a function as fileset predicate...
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.
