make h_errno thread-local

the framework to do this always existed but it was deemed unnecessary
because the only [ex-]standard functions using h_errno were not
thread-safe anyway. however, some of the nonstandard res_* functions
are also supposed to set h_errno to indicate the cause of error, and
were unable to do so because it was not thread-safe. this change is a
prerequisite for fixing them.

src/network/h_errno.c

@@ -1,9 +1,7 @@
 #include <netdb.h>
-
-#undef h_errno
-int h_errno;
+#include "pthread_impl.h"
 
 int *__h_errno_location(void)
 {
-	return &h_errno;
+	return &__pthread_self()->h_errno_val;
 }