Add a variant to the acPreProcForModifyAVUMetadata and acPostProcForModifyAVUMetadata with extra parameters for the new attribute name, value, and unit. This is called when executing an "imeta mod" command. The default rule delegates the PEPs to the old PEPs for backward compatibility

git-svn-id: svn://irodssvn.ucsd.edu/trunk@5528 ea137bd6-aa86-47b6-b90d-b101d9f19a90