Fixed various GCC compiler warnings for the kernel.

Only main() now gives a warning about the return type (GCC wants an int).

kernel/system/irqctl.c

@@ -23,7 +23,6 @@ register message *m_ptr;	/* pointer to request message */
   /* Dismember the request message. */
   int irq_vec;
   int irq_hook_id;
-  int proc_nr;  
   int r = OK;
   irq_hook_t *hook_ptr;
 

kernel/system/proctl.c

@@ -188,8 +188,7 @@ message *m_ptr;			/* pointer to request message */
 {
 /* Handle sys_exit. A user process has exited (the PM sent the request).
  */
-  register struct proc *rp, *rc;
-  struct proc *np, *xp;
+  register struct proc *rc;
   int exit_proc_nr;				
 
   /* Get a pointer to the process that exited. */

kernel/system/sysctl.c

@@ -25,7 +25,6 @@ message *m_ptr;			/* pointer to request message */
 /* Handle sys_exit. A server or driver wants to exit. This may happen
  * on a panic, but also is done when MINIX is shutdown.
  */
-  register struct proc *rp;
   int proc_nr = m_ptr->m_source;	/* can only exit own process */
 
   if (m_ptr->EXIT_STATUS != 0) {