pipicpowerd.c add power up and down script calls

If the file '/usr/sbin/atpwrup' exists it is executed at power up of
the system. The optional script '/usr/sbin/atpwrdown' is executed just
before power down if it exists.

The CPU temperature is now written to the statistics file
'pipicpowers.log'.
pipicpowerd.c update manual page

Add description of 'atpwrup' and 'atpwrdown' scripts to the manual page.