Make sysdig-probe kernel module work with kernel 3.15.0

https://github.com/draios/sysdig/issues/128

It turned out to be not as easy as expected. Tracepoints can now be set only by `struct tracepoint` handle, not by name. The problem is that all tracepoints used by sysdig are not exported with `EXPORT_TRACEPOINT_SYMBOL_GPL` in core kernel. As a workaround I used `for_each_kernel_tracepoint` function to iterate over all tracepoints and store relevant `struct tracepoint` handles in global variables. The handle pointers don't change during kernel lifetime, so it should be safe to do so.