unix, windows: map EHOSTDOWN errno

Fixes: https://github.com/libuv/libuv/issues/195
PR-URL: https://github.com/libuv/libuv/pull/212
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

include/uv-errno.h

@@ -399,4 +399,20 @@
 # define UV__EMLINK (-4032)
 #endif
 
+/* EHOSTDOWN is not visible on BSD-like systems when _POSIX_C_SOURCE is
+ * defined. Fortunately, its value is always 64 so it's possible albeit
+ * icky to hard-code it.
+ */
+#if defined(EHOSTDOWN) && !defined(_WIN32)
+# define UV__EHOSTDOWN (-EHOSTDOWN)
+#elif defined(__APPLE__) || \
+      defined(__DragonFly__) || \
+      defined(__FreeBSD__) || \
+      defined(__NetBSD__) || \
+      defined(__OpenBSD__)
+# define UV__EHOSTDOWN (-64)
+#else
+# define UV__EHOSTDOWN (-4031)
+#endif
+
 #endif /* UV_ERRNO_H_ */

include/uv.h

@@ -138,6 +138,7 @@ extern "C" {
   XX(EOF, "end of file")                                                      \
   XX(ENXIO, "no such device or address")                                      \
   XX(EMLINK, "too many links")                                                \
+  XX(EHOSTDOWN, "host is down")                                               \
 
 #define UV_HANDLE_TYPE_MAP(XX)                                                \
   XX(ASYNC, async)                                                            \