Fix "installer started" detection

Fix "installer started" detection

NoLocalSettings.php tries to detect whether the installer has been
started or not. But this detection has been broken since I4cf237d when a
change was made to stop setting the session name when NO_SESSION is
defined, causing NoLocalSettings.php to be looking at a different
session-cookie than is actually being used by the installer.

The complete fix is twofold:
* Have WebInstaller::startSession() call session_name()
* Have NoLocalSettings.php not call session_name() when PHP's
  session.auto_start configuration setting is set.

Change-Id: I618d11df902b5d1f70e175bc94137621e9195c2f