use __h_errno_location for h_errno

we do not bother making h_errno thread-local since the only interfaces
that use it are inherently non-thread-safe. but still use the
potentially-thread-local ABI to access it just to avoid lock-in.

src/network/h_errno.c

@@ -1 +1,9 @@
+#include <netdb.h>
+
+#undef h_errno;
 int h_errno;
+
+int *__h_errno_location(void)
+{
+	return &h_errno;
+}

src/network/res_query.c

@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include <netdb.h>
 #include "__dns.h"
 #include "libc.h"