support password helper

support password helper

Allows to integrate UI, similar to ssh-askpass, program prompt user
for password and echo result to stdout.

Settings:
 ---
Password Helper /home/alonbl/vpnc/vpnc-getpass
Xauth interactive
 ---

vpn-getpass script for KDE:
 ---
prompt="$1"
exec kdialog --title "vpnc" --password "$prompt";
 ---

vpn-getpass script for KDE with SecurID:
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prompt="$1"
pass="$(kdialog --title "vpnc" --password "$prompt")" || exit 1
otp="$(RSA_SecurID_getpasswd)" || exit 1
echo "${pass}${otp}"
exit 0
 ---

Based on original patch from Alon Bar-Lev
http://lists.unix-ag.uni-kl.de/pipermail/vpnc-devel/2013-December/004039.html
rebased on current HEAD.

Author: Alon Bar-Lev <alon.barlev@gmail.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
git-svn-id: https://svn.unix-ag.uni-kl.de/vpnc/trunk@545 315857ad-0bdb-0310-b42e-dec37551a5f0