libct: move cmsg helpers to new internal/cmsg package

These helpers all make more sense as a self-contained package and moving
them has the added benefit of removing an unneeded libpathrs dependency
(from libcontainer/utils's import of pathrs-lite) from several test
binaries.

Signed-off-by: Aleksa Sarai <aleksa@amutable.com>

libcontainer/utils/cmsg_deprecated.go

@@ -0,0 +1,35 @@
+package utils
+
+import (
+	"os"
+
+	"github.com/opencontainers/runc/internal/cmsg"
+)
+
+// RecvFile waits for a file descriptor to be sent over the given AF_UNIX
+// socket. The file name of the remote file descriptor will be recreated
+// locally (it is sent as non-auxiliary data in the same payload).
+//
+// Deprecated: This method is deprecated and has been moved to an internal
+// package (see [cmsg.RecvFile]). It will be removed in runc 1.6.
+func RecvFile(socket *os.File) (*os.File, error) {
+	return cmsg.RecvFile(socket)
+}
+
+// SendFile sends a file over the given AF_UNIX socket. file.Name() is also
+// included so that if the other end uses RecvFile, the file will have the same
+// name information.
+//
+// Deprecated: This method is deprecated and has been moved to an internal
+// package (see [cmsg.SendFile]). It will be removed in runc 1.6.
+func SendFile(socket, file *os.File) error {
+	return cmsg.SendFile(socket, file)
+}
+
+// SendRawFd sends a specific file descriptor over the given AF_UNIX socket.
+//
+// Deprecated: This method is deprecated and has been moved to an internal
+// package (see [cmsg.SendRawFd]). It will be removed in runc 1.6.
+func SendRawFd(socket *os.File, msg string, fd uintptr) error {
+	return cmsg.SendRawFd(socket, msg, fd)
+}