syscall: add ucred structure for SCM_CREDENTIALS over UNIX sockets.

Working on issue 1101.

R=golang-dev, rsc, adg
CC=golang-dev
https://golang.org/cl/2338042

src/pkg/syscall/types_linux.c

@@ -108,6 +108,7 @@ typedef struct linger $Linger;
 typedef struct iovec $Iovec;
 typedef struct msghdr $Msghdr;
 typedef struct cmsghdr $Cmsghdr;
+typedef struct ucred $Ucred;
 
 enum {
 	$SizeofSockaddrInet4 = sizeof(struct sockaddr_in),
@@ -117,6 +118,7 @@ enum {
 	$SizeofLinger = sizeof(struct linger),
 	$SizeofMsghdr = sizeof(struct msghdr),
 	$SizeofCmsghdr = sizeof(struct cmsghdr),
+	$SizeofUcred = sizeof(struct ucred),
 };
 
 

src/pkg/syscall/ztypes_linux_386.go

@@ -19,6 +19,7 @@ const (
 	SizeofLinger        = 0x8
 	SizeofMsghdr        = 0x1c
 	SizeofCmsghdr       = 0xc
+	SizeofUcred         = 0xc
 	SizeofInotifyEvent  = 0x10
 )
 
@@ -218,6 +219,12 @@ type Cmsghdr struct {
 	Type  int32
 }
 
+type Ucred struct {
+	Pid int32
+	Uid uint32
+	Gid uint32
+}
+
 type InotifyEvent struct {
 	Wd     int32
 	Mask   uint32

src/pkg/syscall/ztypes_linux_amd64.go

@@ -19,6 +19,7 @@ const (
 	SizeofLinger        = 0x8
 	SizeofMsghdr        = 0x38
 	SizeofCmsghdr       = 0x10
+	SizeofUcred         = 0xc
 	SizeofInotifyEvent  = 0x10
 )
 
@@ -220,6 +221,12 @@ type Cmsghdr struct {
 	Type  int32
 }
 
+type Ucred struct {
+	Pid int32
+	Uid uint32
+	Gid uint32
+}
+
 type InotifyEvent struct {
 	Wd     int32
 	Mask   uint32