Fix double forking issue with pkexec

Several system applications who needs pkexec doesn't start from caja. In Mageia, we switch to use of polkit for Mageia drak-tools. One of them is gurpmi, which runs upon clicking on a rpm to install. After migration to polkit we discovered that gurpmi no longer works in Caja. Digging the issue, it reveals that Caja double forks gurpmi.desktop (rpm's set to be open with gurpmi in Caja) . We get "Refusing to render service to dead parents" error. One of our developers, Colin Gutherie created a patch to avoid double forking

https://bugs.mageia.org/show_bug.cgi?id=11184#c57

This pull request fixes https://github.com/mate-desktop/caja/issues/206 also.