Enable a process to find out what the error code was when delivery of an

asynchronous message resulted in an error.

The model here is that:
 - Iff a sender wishes to be notified, the sender MUST check for errors
   BEFORE sending another asynchronous message.

The reason is that in order to remember the error code, we can't clean up
the message table and hence we risk running out of table space. This is
less of a problem when the sender enables notifications only for errors.

common/include/minix/sysutil.h

@@ -68,6 +68,7 @@ _PROTOTYPE( void get_randomness, (struct k_randomness *, int));
 
 #define asynsend(ep, msg) asynsend3(ep, msg, 0)
 _PROTOTYPE( int asynsend3, (endpoint_t ep, message *msg, int flags));
+_PROTOTYPE( int asyn_geterror, (endpoint_t *dst, message *msg, int *err));
 
 #define ASSERT(c) if(!(c)) { panic("%s:%d: assert %s failed", __FILE__, __LINE__, #c); }