windows: add constants for PMTUD socket options

Related documentation:

- https://learn.microsoft.com/en-us/windows/win32/winsock/ipproto-ip-socket-options
- https://learn.microsoft.com/en-us/windows/win32/winsock/ipproto-ipv6-socket-options

Change-Id: I21b23ca815d1d8135ce5724115b9ca23819ea10a
GitHub-Last-Rev: 9054c5c790
GitHub-Pull-Request: golang/sys#245
Reviewed-on: https://go-review.googlesource.com/c/sys/+/654495
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

windows/types_windows.go

@@ -1074,6 +1074,7 @@ const (
 	IP_ADD_MEMBERSHIP  = 0xc
 	IP_DROP_MEMBERSHIP = 0xd
 	IP_PKTINFO         = 0x13
+	IP_MTU_DISCOVER    = 0x47
 
 	IPV6_V6ONLY         = 0x1b
 	IPV6_UNICAST_HOPS   = 0x4
@@ -1083,6 +1084,7 @@ const (
 	IPV6_JOIN_GROUP     = 0xc
 	IPV6_LEAVE_GROUP    = 0xd
 	IPV6_PKTINFO        = 0x13
+	IPV6_MTU_DISCOVER   = 0x47
 
 	MSG_OOB       = 0x1
 	MSG_PEEK      = 0x2
@@ -1132,6 +1134,15 @@ const (
 	WSASYS_STATUS_LEN  = 128
 )
 
+// enum PMTUD_STATE from ws2ipdef.h
+const (
+	IP_PMTUDISC_NOT_SET = 0
+	IP_PMTUDISC_DO      = 1
+	IP_PMTUDISC_DONT    = 2
+	IP_PMTUDISC_PROBE   = 3
+	IP_PMTUDISC_MAX     = 4
+)
+
 type WSABuf struct {
 	Len uint32
 	Buf *byte