readpassword: show where stdin is connected

Should make debugging situations like

	#852
	Empty stdin in mkinitcpio hook

easier.

Examples:

$ echo -n "" | ./gocryptfs -init a
Choose a password for protecting your files.
Reading Password from stdin (connected to "pipe:[749878]")
Got empty Password from stdin

$ ./gocryptfs -init a < /dev/null
Choose a password for protecting your files.
Reading Password from stdin (connected to "/dev/null")
Got empty Password from stdin

$ ./gocryptfs -init a < /dev/zero
Choose a password for protecting your files.
Reading Password from stdin (connected to "/dev/zero")
fatal: maximum password length of 2048 bytes exceeded

$ ./gocryptfs -init a < /dev/full
Choose a password for protecting your files.
Reading Password from stdin (connected to "/dev/full")
fatal: maximum password length of 2048 bytes exceeded

$ jakob@brikett:~/go/src/github.com/rfjakob/gocryptfs$ ./gocryptfs -init a < /dev/urandom
Choose a password for protecting your files.
Reading Password from stdin (connected to "/dev/urandom")

Your master key is:

    4e45a317-595d8a2d-46493a30-97de86ef-
    540c7364-f0acc297-dd6f2592-7d9a5c97

If the gocryptfs.conf file becomes corrupted or you ever forget your password,
there is only one hope for recovery: The master key. Print it to a piece of
paper and store it in a drawer. This message is only printed once.
The gocryptfs filesystem has been created successfully.
You can now mount it using: gocryptfs a MOUNTPOINT

internal/readpassword/read.go

@@ -87,7 +87,14 @@ func readPasswordTerminal(prompt string) ([]byte, error) {
 // readPasswordStdin reads a line from stdin.
 // It exits with a fatal error on read error or empty result.
 func readPasswordStdin(prompt string) ([]byte, error) {
-	tlog.Info.Printf("Reading %s from stdin", prompt)
+	// This should make debugging situations like
+	// https://github.com/rfjakob/gocryptfs/issues/852
+	// easier. Only works on Linux, otherwise shows "?".
+	target, err := os.Readlink("/proc/self/fd/0")
+	if err != nil {
+		target = "?"
+	}
+	tlog.Info.Printf("Reading %s from stdin (connected to %q)", prompt, target)
 	p, err := readLineUnbuffered(os.Stdin)
 	if err != nil {
 		return nil, err