Fix curlwp

curlwp was actually defined twice:

- One as a member of cpu_info struct (ci_curlwp)
- One as a global symbol (curlwp) defined from sys/kern/init_main.c because there was no curlwp macro

I quote init_main.c:
<quote>
struct  lwp *curlwp = &lwp0;
</quote>

Now curlwp is defined as a macro and points to curcpu()->ci_curlwp. This should be good for uniprocessor and multiprocessor.

Thanks Robert Swindells <rjs> for proposing this fix.