f2ser: add ftg:registered_only metadata field to serialized variables

[ChristopherBignamini]

When a field is serialized by using serialbox functions/subroutines belonging to the FTG module, the ftg:registered_only field is added to the variable metadata (with true/false value). This is not done when the FS module is used instead. This causes an error when serialized data are loaded by making use of a mix of FTG and FS functions/subroutines, which is needed (?) when more flexibility is needed wrt standard serialbox usage. Assuming this mix is necessary and assuming changes in serialbox are not possible, the above error (missing ftg:registered_only field) can be easily fixed in f2ser by adding this metadata when we include a new field to a given serialization savepoint.