Adding an option to use PHP Passthru instead of exec

[eranlahav]

My web hosting is LunarPages and I was able to install EE 1.5 on one of their secure servers with ImageMagick by replacing the exec command with passthru. I just had to remove the exec second variable (the array) to make it work.

Please consider making this configurable as I think it will help others with similar limitation.

Thanks.

EL