restore h_errno ABI compatibility with ancient binaries

prior to commit e68c51ac46, h_errno was
actually an external data object not a macro. bring back the symbol,
and use it as the storage for the main thread's h_errno.

technically this still doesn't provide full compatibility if the
application was multithreaded, but at the time there were no res_*
functions (and they did not set h_errno anyway), so any use of h_errno
would have been via thread-unsafe functions. thus a solution that just
fixes single-threaded applications seems acceptable.

src/network/h_errno.c

@@ -1,7 +1,11 @@
 #include <netdb.h>
 #include "pthread_impl.h"
 
+#undef h_errno
+int h_errno;
+
 int *__h_errno_location(void)
 {
+	if (!__pthread_self()->stack) return &h_errno;
 	return &__pthread_self()->h_errno_val;
 }