power: avoid filesystem access if powerd absent
power: avoid filesystem access if powerd absent

On commodity hardware without olpc-powerd, there is unnecessary
filesystem access.

On XO laptop hardware there are unnecessary errors in log for every
object delete:

	ERROR root: Inhibit Suspend: Could not delete file
	/var/run/powerd-inhibit-suspend/1773

	Exception AttributeError: "'NoneType' object has no attribute
	'endswith'" in <bound method PowerManager.__del__ of
	<sugar3.power.PowerManager instance at 0xa15962c>> ignored

The Clock activity in speaking mode is a good reproducer.

Following changes are made:

- move the directory check to __init__, and set self._path to None if
  olpc-powerd is not present,

- on inhibit_suspend, use self._path, which avoids a check of the
  directory,

- on restore_suspend or __del__, avoid a call to os.unlink if
  olpc-powerd is not present.

quozl/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,godiard/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,tchx84/debian-pkg-sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,puneetgkaur/sugar-toolkit-gtk3