Hide password in batch mode

* doc/lispref/minibuf.texi (Intro to Minibuffers, Reading a Password):
Password hiding is available in batch mode, do not mention it in
the exclusion list.  Mention `read-hide-char'. 

* lisp/subr.el (read-passwd): Use `read-hide-char' if non-nil.  Bind it
when calling `read-string'.

* src/minibuf.c (top): Include systty.h.  Declare external
emacs_get_tty and emacs_set_tty.
(Vread_hide_char): New lisp variable.
(read_minibuf_noninteractive): Hide characters with
Vread_hide_char if it is a character.

* src/sysdep.c (emacs_get_tty, emacs_set_tty): Make them externally visible.

Fixes: debbugs:17839

lisp/subr.el

@@ -2016,7 +2016,7 @@ If optional CONFIRM is non-nil, read the password twice to make sure.
 Optional DEFAULT is a default password to use instead of empty input.
 
 This function echoes `.' for each character that the user types.
-Note that in batch mode, the input is not hidden!
+You could let-bind `read-hide-char' to another hiding character, though.
 
 Once the caller uses the password, it can erase the password
 by doing (clear-string STRING)."
@@ -2041,7 +2041,7 @@ by doing (clear-string STRING)."
                                      beg)))
              (dotimes (i (- end beg))
                (put-text-property (+ i beg) (+ 1 i beg)
-                                  'display (string ?.)))))
+                                  'display (string (or read-hide-char ?.))))))
           minibuf)
       (minibuffer-with-setup-hook
           (lambda ()
@@ -2055,12 +2055,9 @@ by doing (clear-string STRING)."
 	    (setq-local show-paren-mode nil)		;bug#16091.
             (add-hook 'after-change-functions hide-chars-fun nil 'local))
         (unwind-protect
-            (let ((enable-recursive-minibuffers t))
-              (read-string
-               (if noninteractive
-                   (format "%s[INPUT WILL NOT BE HIDDEN!] " prompt) ; bug#17839
-                 prompt)
-               nil t default)) ; t = "no history"
+            (let ((enable-recursive-minibuffers t)
+		  (read-hide-char (or read-hide-char ?.)))
+              (read-string prompt nil t default)) ; t = "no history"
           (when (buffer-live-p minibuf)
             (with-current-buffer minibuf
               ;; Not sure why but it seems that there might be cases where the