KCall methods do not depend on m_source and m_type fields

- substituted the use of the m_source message field by
  caller->p_endpoint in kernel calls. It is the same information, just
  passed more intuitively.
  
- the last dependency on m_type field is removed.
  
- do_unused() is substituted by a check for NULL.

- this pretty much removes the depency of kernel calls on the general
  message format. In the future this may be used to pass the kcall
  arguments in a different structure or registers (x86-64, ARM?) The
  kcall number may be passed in a register already.

kernel/system/do_profbuf.c

@@ -26,7 +26,7 @@ PUBLIC int do_profbuf(struct proc * caller, message * m_ptr)
   struct proc *rp;                          
 
   /* Store process name, control struct, table locations. */
-  if(!isokendpt(m_ptr->m_source, &proc_nr))
+  if(!isokendpt(caller->p_endpoint, &proc_nr))
 	return EDEADSRCDST;
 
   if(cprof_procs_no >= NR_SYS_PROCS)