Concurrent enroll-url launches could both pass the enrollment
marker check and run overlapping elevated installs, racing on
LoadOrExchangeAPIKey and overwriting agent.key.
Add an exclusive flock on {configDir}/enrolling.lock for the
full install path and re-check IsEnrolled under that lock so
only one install exchanges a token and configures the device.
Signed-off-by: Ludovic Vielle <ludovic@probo.com>
ae769f52a1