Set approximative system time after power down with date(1).

File '/var/lib/pipicpowerd/pwrup' is created at power down. If this file
exists at power up the PIC internal counter is used to estimate the time
spent in sleep mode. The date(1) is then called to calculate approximative
time after wake up. This is written to file '/var/lib/pipicpowerd/waketime'.
The date(1) is called again to set the system time with value found from
this file.