add EHOSTDOWN errno.

include/errno.h

@@ -112,6 +112,7 @@ extern int errno;		  /* place where the error numbers go */
 #define ENETDOWN      (_SIGN 77)  /* network is down */
 #define EPFNOSUPPORT  (_SIGN 78)  /* Protocol family not supported */
 #define EDESTADDRREQ  (_SIGN 79)  /* Destination address required */
+#define EHOSTDOWN     (_SIGN 80)  /* Host down */
 
 /* The following are not POSIX errors, but they can still happen. 
  * All of these are generated by the kernel and relate to message passing.