implement stdinserver command + corresponding server

How it works:

`zrepl stdinserver CLIENT_IDENTITY`
 * connects to the socket in $global.serve.stdinserver.sockdir/CLIENT_IDENTITY
 * sends its stdin / stdout file descriptors to the `zrepl daemon` process (see cmsg(3))
 * does nothing more

This enables a setup where `zrepl daemon` is not directly exposed to the
internet but instead all traffic is tunnelled through SSH.
The server with the source job has an authorized_keys file entry for the
public key used by the corresponding pull job

 command="/mnt/zrepl stdinserver CLIENT_IDENTITY" ssh-ed25519 AAAAC3NzaC1E... zrepl@pullingserver

cmd/config_serve_stdinserver.go

@@ -0,0 +1,123 @@
+package cmd
+
+import (
+	"github.com/ftrvxmtrx/fd"
+	"github.com/mitchellh/mapstructure"
+	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/util"
+	"io"
+	"net"
+	"os"
+	"path"
+	"path/filepath"
+)
+
+type StdinserverListenerFactory struct {
+	ClientIdentity   string `mapstructure:"client_identity"`
+	ConnLogReadFile  string `mapstructure:"connlog_read_file"`
+	ConnLogWriteFile string `mapstructure:"connlog_write_file"`
+}
+
+func parseStdinserverListenerFactory(i map[string]interface{}) (f *StdinserverListenerFactory, err error) {
+
+	f = &StdinserverListenerFactory{}
+
+	if err = mapstructure.Decode(i, f); err != nil {
+		return nil, errors.Wrap(err, "mapstructure error")
+	}
+	if !(len(f.ClientIdentity) > 0) {
+		err = errors.Errorf("must specify 'client_identity'")
+		return
+	}
+	return
+}
+
+func stdinserverListenerSockpath(clientIdentity string) (addr *net.UnixAddr, err error) {
+	sockpath := path.Join(conf.Global.Serve.Stdinserver.SockDir, clientIdentity)
+	addr, err = net.ResolveUnixAddr("unix", sockpath)
+	if err != nil {
+		return nil, errors.Wrap(err, "cannot resolve unix address")
+	}
+	return addr, nil
+}
+
+func (f *StdinserverListenerFactory) Listen() (al AuthenticatedChannelListener, err error) {
+
+	unixaddr, err := stdinserverListenerSockpath(f.ClientIdentity)
+
+	sockdir := filepath.Dir(unixaddr.Name)
+	sdstat, err := os.Stat(sockdir)
+	if err != nil {
+		return nil, errors.Wrapf(err, "cannot stat(2) sockdir '%s'", sockdir)
+	}
+	if !sdstat.IsDir() {
+		return nil, errors.Errorf("sockdir is not a directory: %s", sockdir)
+	}
+	p := sdstat.Mode().Perm()
+	if p&0007 != 0 {
+		return nil, errors.Errorf("sockdir must not be world-accessible (permissions are %#o)", p)
+	}
+
+	ul, err := net.ListenUnix("unix", unixaddr) // TODO
+	if err != nil {
+		return nil, errors.Wrapf(err, "cannot listen on unix socket %s", unixaddr)
+	}
+
+	l := &StdinserverListener{ul, f.ConnLogReadFile, f.ConnLogWriteFile}
+
+	return l, nil
+}
+
+type StdinserverListener struct {
+	l                *net.UnixListener
+	ConnLogReadFile  string `mapstructure:"connlog_read_file"`
+	ConnLogWriteFile string `mapstructure:"connlog_write_file"`
+}
+
+type fdRWC struct {
+	stdin, stdout *os.File
+	control       *net.UnixConn
+}
+
+func (f fdRWC) Read(p []byte) (n int, err error) {
+	return f.stdin.Read(p)
+}
+
+func (f fdRWC) Write(p []byte) (n int, err error) {
+	return f.stdout.Write(p)
+}
+
+func (f fdRWC) Close() (err error) {
+	f.stdin.Close()
+	f.stdout.Close()
+	return f.control.Close()
+}
+
+func (l *StdinserverListener) Accept() (ch io.ReadWriteCloser, err error) {
+	c, err := l.l.Accept()
+	if err != nil {
+		err = errors.Wrap(err, "error accepting on unix listener")
+		return
+	}
+
+	// Read the stdin and stdout of the stdinserver command
+	files, err := fd.Get(c.(*net.UnixConn), 2, []string{"stdin", "stdout"})
+	if err != nil {
+		err = errors.Wrap(err, "error receiving fds from stdinserver command")
+		c.Close()
+	}
+
+	rwc := fdRWC{files[0], files[1], c.(*net.UnixConn)}
+
+	rwclog, err := util.NewReadWriteCloserLogger(rwc, l.ConnLogReadFile, l.ConnLogWriteFile)
+	if err != nil {
+		panic(err)
+	}
+
+	return rwclog, nil
+
+}
+
+func (l *StdinserverListener) Close() (err error) {
+	return l.l.Close() // removes socket file automatically
+}