field_allowed_in(A|B, C) returns true only if the field is allowed in C and either A or B.
Undef is returned for non-official fields.
Official fields are copied only if the field is allowed in both types of objects. Custom fields are treated in a specific manner. When the target is not among CTRL_PKG_SRC, CTRL_PKG_DEB or CTRL_FILE_CHANGES, then they are alway copied as is (the X- prefix is kept). Otherwise they are not copied except if the target object matches the target destination encoded in the field name. The initial X denoting custom fields can be followed by one or more letters among ``S'' (Source: corresponds to CTRL_PKG_SRC), ``B'' (Binary: corresponds to CTRL_PKG_DEB) or ``C'' (Changes: corresponds to CTRL_FILE_CHANGES).
Returns undef if nothing has been copied or the name of the new field added to $to otherwise.
Returns the list of fields that have been added to $to.