sadc: Add a -f flag to force fdatasync() use

sadc: Add a -f flag to force fdatasync() use

For quite some time, the sadc utility has not used fdatasync() when writing
stat information to disk. This resulted in instances where data files could
be corrupted or entries lost if a system encountered a sudden reset
condition. This change adds a "-f" flag which can be used to bring back the
previous behaviour if end users require it.

Note, the fdatasync() lowers the likelihood of lost data, but does so at
the expense of performance within the write operation.