glib: Create configuration files with 0666 mode again
Commit 3b6460b9 changed various calls from g_file_set_contents to g_file_set_contents_full. Along with this change the the file mode changed from g_file_set_contents's default of 0666, to 0600.
Using 0600 mode don't typically have much impact, but aside from 0666 (typically dropped to 0644 by umask) being more common for configuration files, it also avoids the small annoyance of some files such as mimeapps.list constantly switching modes (xdg-mime sets it to 0666 and glib sets it to 0600).
Reverts file mode changes from commit 3b6460b9, other usages of 0600 mode are intentional (locks, temp. files).