SMART — how to deal with unimplemented files

For now it is unclear how to deal with records in system-files with functions that are not implemented. Thus in Section 2.4 the following should be added:
"If a file is not implemented there is no need to implement the respective header record; the "NoFile"-error is returned instead. Unimplemented records in the middle of a file should return 0x00 in order to prevent holes in the IFS (the "NoRecord"-error is applicable for records beyond the length of a file only)."

Accept changes as proposed.