Hide password in batch mode

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