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_vtimer.c

@@ -34,7 +34,7 @@ PUBLIC int do_vtimer(struct proc * caller, message * m_ptr)
       return(EINVAL);
 
   /* The target process must be valid. */
-  proc_nr_e = (m_ptr->VT_ENDPT == SELF) ? m_ptr->m_source : m_ptr->VT_ENDPT;
+  proc_nr_e = (m_ptr->VT_ENDPT == SELF) ? caller->p_endpoint : m_ptr->VT_ENDPT;
   if (!isokendpt(proc_nr_e, &proc_nr)) return(EINVAL);
   rp = proc_addr(proc_nr);