Added a new multi-user target for targets esame*, so we can avoid to start NFS services if not needed

The first commit (the one that was reverted) contained a spurious,
unrelated change.

This reverts commit 2ed8cbd1.

The bta-no-usbpen was a link to /bin/sleep.

Passaggio ai target secondari (esame e graphical) attraverso comando 'start' invece di 'isolate' (NetworkManager non ripartiva più dopo il passaggio di target)

Forgot to add esame-get-config.timer to esameshow.target and the target itself in the install script.

New feature (fix for someone...): don't erase user's home if the computer crashes or reboots during the exam.
The user's home is erased (if required) only when the exam mode starts the first time.

Added esamesync-esame.service to graphical.target requirements, so the boot process stops if the master server is not reachable.

Otherwise we would get something like this:

  useradd: user 'rdesktop' already exists
  Error creating user rdesktop
  User rdesktop was created

Removed esame-get-config-boot.service as a requirement from some services (it is not needed anymore as esame-configurator target is reached before any other target at boot time).