Open an existing key file.
Open an existing key file.If update is 0, the file is opened for read access only, if update is not 0 the file is opened for reading and writing
A file set tag can be attached to the file, if a file having a non NULL file set tag is flushed to disk all files having the same file set tag attached are flushed as well.
RESTRICTIONS:
- the file must exist already
- the current record of the file will not be set