clean way howto use clib4

@joerg

I have a working vfork for AmigaOS4...
so now its not only MorphOS that has it...

the stack is moved, so never do &local to get the address, before the vfork(), besides closing BPTR file pointers, and doing stuff like that will be like begging for problems and bugs. a lot of stuff must be atomic..
vfork is messy not so easy to see where the vfork thread ends.

I guess pthreads spawn is safer, easier to read. and less likely to break on a system update. (in anycase the way I did it might be quite system friendly... besides the side effects.), unless the OS is updated to 64bit, or we change to a different CPU type.