Kernel: clean up include statements a bit

Coverity was flagging a recursive include between kernel.h and
cpulocals.h. As cpulocals.h also included proc.h, we can move that
include statement into kernel.h, and clean up the source files'
include statements accordingly.

kernel/system.h

@@ -30,11 +30,7 @@
 #ifndef SYSTEM_H
 #define SYSTEM_H
 
-/* Common includes for the system library. */
-#include "debug.h"
 #include "kernel.h"
-#include "proto.h"
-#include "proc.h"
 
 int do_exec(struct proc * caller, message *m_ptr);
 #if ! USE_EXEC