[PATCH] getpass: Don't fail if stdin is not a tty

[PATCH] getpass: Don't fail if stdin is not a tty

We don't need to manipulate the tty state (such as turning off
echo) when prompting for passwords if we're not reading from a tty.

Backport based on a patch provided by Stef Walter <stefw@gnome.org>.

Fix bug #9767 - "net ads join" fails when called via stdin.

Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Thu Apr 11 11:35:07 CEST 2013 on sn-devel-104