Summary:
Add a command line option to start edenfs only if there is at least one
Eden checkout already configured.
This makes it easier to automatically start edenfs on system boot only if the
user needs edenfs.
Reviewed By: wez
Differential Revision: D8174819
fbshipit-source-id: bf585b8c5aa719d3b6424567f2e21c1c63850d66
