Bug#324779: apache2-mpm-worker should check whether apache2 is installed

[Bryan Klingner <bryan@overt.org>]

Package: apache2-mpm-worker
Version: 2.0.54-4

I installed apache2, which caused apache2-mpm-worker to be installed.  
I then removed apache2, which did not cause apache2-mpm-worker to be  
removed. When I tried to upgrade it or remove it, it tried to invoke  
(from line 8 of it's prerm script):

/usr/sbin/invoke-rc.d apache2 stop

But since apache2 was not there, the stop failed, and it could not be  
removed. I had to manually edit the prerm script to remove it. This  
might also be an issue with other apache2 friends.

Thanks so much for the hard work!

Bryan Klingner