system_init() renamed to arch_init()

- a better name for architecture specific init function

- some of x86 init code must execute in protected mode

- prot_init() removed from this function and still called in cstart() Imho this
  should be called from the architecture specific assembly not cstart. cstart
  perform Minix monitor specific tasks and will be touched once another
  bootloader is in use, e.g. booting via tftp, therefore we keep it as is for
  now.

- this is a backport from the SMP code which requires this. Merging will be simpler

kernel/start.c

@@ -29,8 +29,8 @@ U16_t parmoff, parmsize;	/* boot parameters offset and length */
   kinfo.data_base = seg2phys(ds);
   kinfo.data_size = (phys_bytes) &end;		/* size of data segment */
 
-  /* Architecture-dependent initialization. */
-  system_init();
+  /* protection initialization */
+  prot_init();
 
   /* Copy the boot parameters to the local buffer. */
   arch_get_params(params_buffer, sizeof(params_buffer));