revised low level system call convention

We'll see how well this one works. Now all system calls pass arguments
via 4 registers (thus max 4 parameters to system calls). Obviously, if
a call only takes 2 parameters, the other 2 will just be ignored. All
system calls pass through x86_syscall as their entrance to the kernel
and from there get dispatched to the appropriate handler by means of
a lookup table: syscall_table.