guest additions initscripts should be more flexible

[Timothe Litt]

Adding options to the virtualbox guest additions startup requires editing the init script. This is a pain, especially when installing from source or creating a new install. Note that with systemd the init file isn't in /etc/init.d, but rather /opt/VBoxGuestAdditions-$VERSION/init - which is refreshed on an install/upgrade.

Most init scripts separate the startup script from options by providing a file in /etc/default or /etc/sysconfig.

e.g. This should work for most distributions conventions

/etc/sysconfig:
OPTIONS="--disable-timesync"

https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Additions/linux/installer/vboxadd-service.sh

[ -f /etc/default/vboxadd-service ] && . /etc/default/vboxadd-service
[ -f /etc/sysconfig/vboxadd-service ] && . /etc/sysconfig/vboxadd-service

and 
         vboxaddrunning || {
             echo "VirtualBox Additions module not loaded!"
             exit 1
         }
         testbinary
-        daemon $binary --pidfile $PIDFILE > /dev/null
+        daemon $binary --pidfile $PIDFILE $OPTIONS > /dev/null
         RETVAL=$?
         succ_msg
     fi

     return $RETVAL