syscall: reduce duplication between *bsd and linux

See discussion: https://groups.google.com/forum/#!topic/golang-dev/zSmH0lQxKAs

Part 1 of several.

Move identical types and functions to syscall_unix.go.

R=rsc, mikioh.mikioh, r
CC=golang-dev
https://golang.org/cl/10392048
diff --git a/src/pkg/syscall/syscall_unix.go b/src/pkg/syscall/syscall_unix.go
index f29395b..0432b49 100644
--- a/src/pkg/syscall/syscall_unix.go
+++ b/src/pkg/syscall/syscall_unix.go
@@ -152,6 +152,66 @@
 	return
 }
 
+// For testing: clients can set this flag to force
+// creation of IPv6 sockets to return EAFNOSUPPORT.
+var SocketDisableIPv6 bool
+
+type Sockaddr interface {
+	sockaddr() (ptr uintptr, len _Socklen, err error) // lowercase; only we can define Sockaddrs
+}
+
+type SockaddrInet4 struct {
+	Port int
+	Addr [4]byte
+	raw  RawSockaddrInet4
+}
+
+type SockaddrInet6 struct {
+	Port   int
+	ZoneId uint32
+	Addr   [16]byte
+	raw    RawSockaddrInet6
+}
+
+type SockaddrUnix struct {
+	Name string
+	raw  RawSockaddrUnix
+}
+
+func Bind(fd int, sa Sockaddr) (err error) {
+	ptr, n, err := sa.sockaddr()
+	if err != nil {
+		return err
+	}
+	return bind(fd, ptr, n)
+}
+
+func Connect(fd int, sa Sockaddr) (err error) {
+	ptr, n, err := sa.sockaddr()
+	if err != nil {
+		return err
+	}
+	return connect(fd, ptr, n)
+}
+
+func Socket(domain, typ, proto int) (fd int, err error) {
+	if domain == AF_INET6 && SocketDisableIPv6 {
+		return -1, EAFNOSUPPORT
+	}
+	fd, err = socket(domain, typ, proto)
+	return
+}
+
+func Socketpair(domain, typ, proto int) (fd [2]int, err error) {
+	var fdx [2]int32
+	err = socketpair(domain, typ, proto, &fdx)
+	if err == nil {
+		fd[0] = int(fdx[0])
+		fd[1] = int(fdx[1])
+	}
+	return
+}
+
 func Sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	if raceenabled {
 		raceReleaseMerge(unsafe.Pointer(&ioSync))