tree-wide: add helper for IPv4/IPv6 sockopts

A variety of sockopts exist both for IPv4 and IPv6 but require a
different pair of sockopt level/option number. Let's add helpers for
these that internally determine the right sockopt to call.

This should shorten code that generically wants to support both ipv4 +
ipv6 and for the first time adds correct support for some cases where we
only called the ipv4 versions, and not the ipv6 options.

src/basic/missing_network.h

@@ -6,6 +6,11 @@
 #define IPV6_UNICAST_IF 76
 #endif
 
+/* linux/in6.h or netinet/in.h */
+#ifndef IPV6_TRANSPARENT
+#define IPV6_TRANSPARENT 75
+#endif
+
 /* Not exposed but defined at include/net/ip.h */
 #ifndef IPV4_MIN_MTU
 #define IPV4_MIN_MTU 68