resolv: Introduce struct resolv_context [BZ #21668]

struct resolv_context objects provide a temporary resolver context
which does not change during a name lookup operation.  Only when the
outmost context is created, the stub resolver configuration is
verified to be current (at present, only against previous res_init
calls).  Subsequent attempts to obtain the context will reuse the
result of the initial verification operation.

struct resolv_context can also be extended in the future to store
data which needs to be deallocated during thread cancellation.

resolv/res-close.c

@@ -83,6 +83,7 @@
  */
 
 #include <resolv-internal.h>
+#include <resolv_context.h>
 #include <not-cancel.h>
 
 /* Close all open sockets.  If FREE_ADDR is true, deallocate any
@@ -124,6 +125,8 @@ libc_hidden_def (__res_nclose)
 static void __attribute__ ((section ("__libc_thread_freeres_fn")))
 res_thread_freeres (void)
 {
+  __resolv_context_freeres ();
+
   if (_res.nscount == 0)
     /* Never called res_ninit.  */
     return;