No linear addresses in message delivery

- removes p_delivermsg_lin item from the process structure and code
  related to it

- as the send part, the receive does not need to use the
  PHYS_COPY_CATCH() and umap_local() couple.  

- The address space of the target process is installed before
  delivermsg() is called.

- unlike the linear address, the virtual address does not change when
  paging is turned on nor after fork().

kernel/system/do_fork.c

@@ -109,7 +109,6 @@ PUBLIC int do_fork(struct proc * caller, message * m_ptr)
 
   /* Install new map */
   r = newmap(caller, rpc, map_ptr);
-  FIXLINMSG(rpc);
 
   /* Don't schedule process in VM mode until it has a new pagetable. */
   if(m_ptr->PR_FORK_FLAGS & PFF_VMINHIBIT) {