Refactor ac(Pre)PostProcForModifyAVUMetadata into three rules for different imeta commands and make sure that the arguments align with the parameters, based on Tony's suggestions. The three rules are:
4 arguments: cp
6 arguments: add, mod, set, rm
9 arguments: mod

git-svn-id: svn://irodssvn.ucsd.edu/trunk@5529 ea137bd6-aa86-47b6-b90d-b101d9f19a90