Bugfix: Invalid seeked value on failure in f_file_seek().

Bugfix: Invalid seeked value on failure in f_file_seek().

The `lseek()` can return a negative value.

Use a local variable to get the `lseek()` value.
This then allows for the `seeked` to be NULL.

Change the `seeked` parameter to be optional and when set to NULL then it is not assigned.
This neither breaks API nor ABI.

The `seeked` value is only assigned when the result is `-1`.
The POSIX standard is inconsistent about negative return values.
In the case of `lseek()`, only `-1` is technically an error.