In some places we use g_file_set_contents() after a umask() to limit
the permissions of the created file. Unfortunately if the containing
directory has a default ACL the umask will be ignored and the new file
will have a mode equal to the default ACL (since g_file_set_contents()
opens the file with mode 0666).

Calling a chmod() after the file gets created is insecure (see commit
60b7ed3b) and so the only solution seems to be to reimplement
g_file_set_contents() and accept a mode as parameter.

We already had similar functions in the tree, consolidate them into a
new generic utility function.

https://bugzilla.gnome.org/show_bug.cgi?id=769702