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/main.c

@@ -37,6 +37,9 @@ PUBLIC void main()
   reg_t ktsb;			/* kernel task stack base */
   struct exec e_hdr;		/* for a copy of an a.out header */
 
+   /* Architecture-dependent initialization. */
+   arch_init();
+ 
   /* Clear the process table. Anounce each slot as empty and set up mappings 
    * for proc_addr() and proc_nr() macros. Do the same for the table with 
    * privilege structures for the system processes.