0 is not a system call (thanks to Peter Froehlich)

Previously, these were inconsistent: they used their struct proc
argument for bounds checking, but always copied the argument from the
current address space (and hence the current process).  Drop the
struct proc argument and always use the current proc.

Suggested by Carmi Merimovich.

Cut section about commentary
Add (failing) link to xv6 source section pointing to the text.
Delete the incorrect/incomplete list of universities using jos/xv6

This way, the bootloader doesn't have to translate the entry point.
This also makes xv6 multiboot-compliant and follows the convention
used by Linux.