Implement second fallback mode for DNS requests.

There is some more shardware/software out there which has problems
if two DNS requests are sent using the same tuple

  (source addr, source port, dest addr, dest port)

This can range from firewalls to load balancers.  Some of the vendors
already fixed it in response to this problem.  Still, we need a way
to make glibc work with broken environments.  The single-request-reopen
flag can be used or we fall back automatically to this mode.

resolv/resolv.h

@@ -216,6 +216,8 @@ struct res_sym {
 					   reverse lookup */
 #define RES_USE_EDNS0	0x00100000	/* Use EDNS0.  */
 #define RES_SNGLKUP	0x00200000	/* one outstanding request at a time */
+#define RES_SNGLKUPREOP	0x00400000	/* -"-, but open new socket for each
+					   request */
 
 #define RES_DEFAULT	(RES_RECURSE|RES_DEFNAMES|RES_DNSRCH|RES_NOIP6DOTINT)