add cli command "mox admin imapserve $preauthaddress"

for admins to open an imap connection preauthenticated for an account (by address), also when
it is disabled for logins.

useful for migrations. the admin typically doesn't know the password of the
account, so couldn't get an imap session (for synchronizing) before.

tested with "mox localserve" and running:

	mutt -e 'set tunnel="MOXCONF=/home/mjl/.config/mox-localserve/mox.conf ./mox admin imapserve mox@localhost"'

may also work with interimap, but untested.

i initially assumed imap would be done fully on file descriptor 0, but mutt
expects imap output on fd 1. that's the default now. flag -fd0 is for others
that expect it on fd0.

for issue #175, suggested by DanielG

main.go

@@ -171,6 +171,8 @@ var commands = []struct {
 	{"config ensureacmehostprivatekeys", cmdConfigEnsureACMEHostprivatekeys},
 	{"config example", cmdConfigExample},
 
+	{"admin imapserve", cmdIMAPServe},
+
 	{"checkupdate", cmdCheckupdate},
 	{"cid", cmdCid},
 	{"clientconfig", cmdClientConfig},
@@ -3720,6 +3722,58 @@ func ctlcmdReassignthreads(ctl *ctl, account string) {
 	ctl.xstreamto(os.Stdout)
 }
 
+func cmdIMAPServe(c *cmd) {
+	c.params = "preauth-address"
+	c.help = `Initiate a preauthenticated IMAP connection on file descriptor 0.
+
+For use with tools that can do IMAP over tunneled connections, e.g. with SSH
+during migrations. TLS is not possible on the connection, and authentication
+does not require TLS.
+`
+	var fd0 bool
+	c.flag.BoolVar(&fd0, "fd0", false, "write IMAP to file descriptor 0 instead of stdout")
+	args := c.Parse()
+	if len(args) != 1 {
+		c.Usage()
+	}
+
+	address := args[0]
+	output := os.Stdout
+	if fd0 {
+		output = os.Stdout
+	}
+	ctlcmdIMAPServe(xctl(), address, os.Stdin, output)
+}
+
+func ctlcmdIMAPServe(ctl *ctl, address string, input io.ReadCloser, output io.WriteCloser) {
+	ctl.xwrite("imapserve")
+	ctl.xwrite(address)
+	ctl.xreadok()
+
+	done := make(chan struct{}, 1)
+	go func() {
+		defer func() {
+			done <- struct{}{}
+		}()
+		_, err := io.Copy(output, ctl.conn)
+		if err == nil {
+			err = io.EOF
+		}
+		log.Printf("reading from imap: %v", err)
+	}()
+	go func() {
+		defer func() {
+			done <- struct{}{}
+		}()
+		_, err := io.Copy(ctl.conn, input)
+		if err == nil {
+			err = io.EOF
+		}
+		log.Printf("writing to imap: %v", err)
+	}()
+	<-done
+}
+
 func cmdReadmessages(c *cmd) {
 	c.unlisted = true
 	c.params = "datadir account ..."