unix: implement L2TPIP socket address on Linux

Add constants and types to support IP-encapsulated L2TP/RFC3931
tunnels on Linux systems.

The L2TP subsystem for IP encapsulated tunnels hooks into the inet
kernel code using a specific IP protocol value.  In order to handle
this, anyToSockaddr now has to query the socket protocol type using
GetsockoptInt for the AF_INET and AF_INET6 address families.

Although this change is reasonably simple, unit tests have been added
to validate handling of the new types.

Fixes golang/go#37787

Change-Id: I16ae1e24dcced4ccc6ce6a79a90a5a2f6a560967
GitHub-Last-Rev: ca554ad1b6d3a5e55c70f05cf0e542968fa2418c
GitHub-Pull-Request: golang/sys#60
Reviewed-on: https://go-review.googlesource.com/c/sys/+/223157
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/types.go

@@ -205,6 +205,8 @@ union sockaddr_all {
 	struct sockaddr_ll s5;
 	struct sockaddr_nl s6;
 	struct sockaddr_pppox s7;
+	struct sockaddr_l2tpip s8;
+	struct sockaddr_l2tpip6 s9;
 };
 
 struct sockaddr_any {
@@ -509,6 +511,10 @@ type RawSockaddrPPPoX [C.sizeof_struct_sockaddr_pppox]byte
 
 type RawSockaddrTIPC C.struct_sockaddr_tipc
 
+type RawSockaddrL2TPIP C.struct_sockaddr_l2tpip
+
+type RawSockaddrL2TPIP6 C.struct_sockaddr_l2tpip6
+
 type RawSockaddr C.struct_sockaddr
 
 type RawSockaddrAny C.struct_sockaddr_any
@@ -561,6 +567,8 @@ const (
 	SizeofSockaddrXDP       = C.sizeof_struct_sockaddr_xdp
 	SizeofSockaddrPPPoX     = C.sizeof_struct_sockaddr_pppox
 	SizeofSockaddrTIPC      = C.sizeof_struct_sockaddr_tipc
+	SizeofSockaddrL2TPIP    = C.sizeof_struct_sockaddr_l2tpip
+	SizeofSockaddrL2TPIP6   = C.sizeof_struct_sockaddr_l2tpip6
 	SizeofLinger            = C.sizeof_struct_linger
 	SizeofIovec             = C.sizeof_struct_iovec
 	SizeofIPMreq            = C.sizeof_struct_ip_mreq