use dh_installinit to handle Debian start scripts

the Debian package build (Debian, Ubuntu, Univention) mechanism
offers support functions to handle daemon init scripts.
This patch uses this mechanism.
Changed behaviour:
  bareos-fd and bareos-sd are started on install.
    This is normally not useful, because they are not configured correctly.
    However, is the Debian default behavior.
  bareos-fd and bareos-sd are restarted on update.
  bareos-fd, bareos-sd and bareos-dir are stopped on removal.
  When doing an install from source (make, make install) on Debian,
    init scripts will not be installed.
    An extra call "make install-autostart" is required.
configure