When starting nautilus for the first time, it creates the ~/Desktop folder
and copies the starthere.desktop file inside it.  The problem is that the
copy gets the same permissions as the original file.  In (at least) the
pkgsrc packaging system, system wide files get installed with restrictive
permissions (no write), so starthere.desktop gets 444 mode when copied in
~/Desktop.

This results in the "Start Here" icon having the "write denied" icon over
it, which looks ugly after the first bootup.  The solution is to change the
permissions of the copied file to 644, to ensure we get something sane.

At first, I used system(2) to call /bin/chmod, but then though that
directly using chmod(2) could be better.  So, the attached patch does the
later.