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_vdevio.c

@@ -94,7 +94,7 @@ PUBLIC int do_vdevio(struct proc * caller, message * m_ptr)
 		{
 			printf(
 		"do_vdevio: I/O port check failed for proc %d, port 0x%x\n",
-				m_ptr->m_source, port);
+				caller->p_endpoint, port);
 			return EPERM;
 		}
 	}