Remove crypto/tls dependency

It appears that when we import github.com/coreos/go-systemd/activation,
it brings in the whole crypto/tls package (which is not used by runc
directly or indirectly), making the runc binary size larger and
potentially creating issues with FIPS compliance.

Let's copy the code of function we use from go-systemd/activation
to avoid that.

The space savings are:

$ size runc.before runc.after
   text	   data	    bss	    dec	    hex	filename
7101084	5049593	 271560	12422237	 bd8c5d	runc.before
6508796	4623281	 229128	11361205	 ad5bb5	runc.after

Reported-by: Dimitri John Ledkov <dimitri.ledkov@surgut.co.uk>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

internal/third_party/systemd/activation/files_unix.go

@@ -0,0 +1,76 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !windows
+
+// Package activation implements primitives for systemd socket activation.
+//
+// It is a partial copy of https://github.com/coreos/go-systemd/v22/activation
+// (https://github.com/coreos/go-systemd/blob/ce60782c0aabb616faa8e60f91e639d91f631e99/activation/files_unix.go),
+// to avoid bringing in crypto/tls dependency.
+package activation
+
+import (
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+const (
+	// listenFdsStart corresponds to `SD_LISTEN_FDS_START`.
+	listenFdsStart = 3
+)
+
+// Files returns a slice containing a `os.File` object for each
+// file descriptor passed to this process via systemd fd-passing protocol.
+//
+// The order of the file descriptors is preserved in the returned slice.
+// `unsetEnv` is typically set to `true` in order to avoid clashes in
+// fd usage and to avoid leaking environment flags to child processes.
+func Files(unsetEnv bool) []*os.File {
+	if unsetEnv {
+		defer func() {
+			// Unsetenv implementation for unix never returns an error.
+			_ = os.Unsetenv("LISTEN_PID")
+			_ = os.Unsetenv("LISTEN_FDS")
+			_ = os.Unsetenv("LISTEN_FDNAMES")
+		}()
+	}
+
+	pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
+	if err != nil || pid != os.Getpid() {
+		return nil
+	}
+
+	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+	if err != nil || nfds <= 0 {
+		return nil
+	}
+
+	names := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
+
+	files := make([]*os.File, 0, nfds)
+	for fd := listenFdsStart; fd < listenFdsStart+nfds; fd++ {
+		syscall.CloseOnExec(fd)
+		name := "LISTEN_FD_" + strconv.Itoa(fd)
+		offset := fd - listenFdsStart
+		if offset < len(names) && len(names[offset]) > 0 {
+			name = names[offset]
+		}
+		files = append(files, os.NewFile(uintptr(fd), name))
+	}
+
+	return files
+}