Re-vendor, and use mtrmac/image-spec:id-based-loader to fix tests

Anyone running (vndr) currently ends up with failing tests in OCI schema
validation because gojsonschema has fixed its "$ref" interpretation, exposing
inconsistent URI usage inside image-spec/schema.

So, this runs (vndr), and uses mtrmac/image-spec:id-based-loader
( https://github.com/opencontainers/image-spec/pull/739 ) to make the tests pass
again.  As soon as that PR is merged we should revert to using the upstream
image-spec repo again.

vendor/golang.org/x/sys/unix/syscall_unix.go

@@ -50,6 +50,16 @@ func errnoErr(e syscall.Errno) error {
 	return e
 }
 
+// clen returns the index of the first NULL byte in n or len(n) if n contains no NULL byte.
+func clen(n []byte) int {
+	for i := 0; i < len(n); i++ {
+		if n[i] == 0 {
+			return i
+		}
+	}
+	return len(n)
+}
+
 // Mmap manager, for use by operating system-specific implementations.
 
 type mmapper struct {
@@ -138,16 +148,19 @@ func Write(fd int, p []byte) (n int, err error) {
 // creation of IPv6 sockets to return EAFNOSUPPORT.
 var SocketDisableIPv6 bool
 
+// Sockaddr represents a socket address.
 type Sockaddr interface {
 	sockaddr() (ptr unsafe.Pointer, len _Socklen, err error) // lowercase; only we can define Sockaddrs
 }
 
+// SockaddrInet4 implements the Sockaddr interface for AF_INET type sockets.
 type SockaddrInet4 struct {
 	Port int
 	Addr [4]byte
 	raw  RawSockaddrInet4
 }
 
+// SockaddrInet6 implements the Sockaddr interface for AF_INET6 type sockets.
 type SockaddrInet6 struct {
 	Port   int
 	ZoneId uint32
@@ -155,6 +168,7 @@ type SockaddrInet6 struct {
 	raw    RawSockaddrInet6
 }
 
+// SockaddrUnix implements the Sockaddr interface for AF_UNIX type sockets.
 type SockaddrUnix struct {
 	Name string
 	raw  RawSockaddrUnix