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.

src/sugar3/power.py

@@ -38,7 +38,10 @@ class PowerManager():
 
     def __init__(self):
         self._suspend_inhibit_counter = 0
+        if os.path.exists(_POWERD_INHIBIT_DIR):
             self._path = os.path.join(_POWERD_INHIBIT_DIR, str(os.getpid()))
+        else:
+            self._path = None
 
     def __del__(self):
         self._remove_flag_file()
@@ -47,10 +50,7 @@ class PowerManager():
         return True
 
     def inhibit_suspend(self):
-        if not os.path.exists(_POWERD_INHIBIT_DIR):
-            return
-
-        if self._suspend_inhibit_counter == 0:
+        if self._path and self._suspend_inhibit_counter == 0:
             try:
                 with open(self._path, 'w') as flag_file:
                     flag_file.write('')
@@ -77,6 +77,7 @@ class PowerManager():
         self._remove_flag_file()
 
     def _remove_flag_file(self):
+        if self._path:
             try:
                 os.unlink(self._path)
             except OSError: