Tightening symbol exports

>for example, the exporting of sys_open() is an active inducement for developers to open files
directly inside the kernel, which is almost never a good idea.

sys_open ha - if you really were to open files within the kernel, then better with
filp_open(), as it takes a kernelspace string and does not need the set_ds() cruft. Though,
you will have to assign the fd yourself - that is, IFF you plan to do that, and in most cases
(of the cases that you even /want/ to open files in the kernel), you do not.