Add --hide-sn parameter

[Napsty]

This PR adds a new parameter --hide-sn to hide the serial number of the drive(s).

root@nas:~/check_smart# ./check_smart.pl -d /dev/sdf -i auto --hide-sn
OK: Drive  WDC WD20SPZX-22CRAT0 S/N <HIDDEN>: no SMART errors detected. |Raw_Read_Error_Rate=0 Spin_Up_Time=1850 Start_Stop_Count=8 Reallocated_Sector_Ct=0 Seek_Error_Rate=0 Power_On_Hours=38056 Spin_Retry_Count=0 Calibration_Retry_Count=0 Power_Cycle_Count=6 Power-Off_Retract_Count=4 Load_Cycle_Count=1239994 Temperature_Celsius=29 Reallocated_Event_Count=0 Current_Pending_Sector=0 Offline_Uncorrectable=0 UDMA_CRC_Error_Count=0 Multi_Zone_Error_Rate=0