Bug#798430: apache2: please add systemd service file

[Stefan Fritsch <sf@sfritsch.de>]

This is not that easy because of all the logic that we have in the 
init script.

One part is about starting/stopping htcacheclean if mod_cache_disk is 
enabled. Maybe instead of doing this check at apache2 startup, this 
could be split into a separate service and a2enmod could active the 
htcacheclean service when mod_cache_disk is enabled.

Another part is that apache2 is not very good at telling its status 
via an exit code. For example, when doing reload/stop/graceful-stop, 
the controlling apache2 process will only send a signal and not wait 
for the result of the operation. I don't know enough about systemd to 
tell if this can be easily done via unit files.

Also, some configuration is done via /etc/apache2/envvars and 
/etc/default/apache2 . This would need to be taken care of, too.