exec enlightenment instead of enlightenment_start

exec enlightenment instead of enlightenment_start

    Refer to below commit of mainstream. enlightenment_start can't
    be used to start enlightenment in systemd.

    -------------------------------------------------------
    commit 6f955263fa76649682066adc82f12d4249d9c3d0
    Author: Cedric BAIL <cedric.bail@samsung.com>
    Commit: Cedric BAIL <cedric.bail@samsung.com>

    enlightenment: do not propagate the systemd socket to E17.

    In a proper systemd user session setup, enlightenment_start will be
    gone. That is still not doable today and as I plan to roll more feature
    in EFL regarding systemd, we need to prevent enlightenment to believe
    it is the process started by systemd.

Change-Id: I8cfcef9abf9318da3babd96c7f9191796fe0fabb