When auth-user-pass file has no password query the management interface

When auth-user-pass file has no password query the management interface
(if available).

When only username is found in the file, redirect the auth-user-pass
query to the management interface if management-query-passwords is
enabled.  Otherwise the user is prompted on console, if available,
as before.

This changes the behaviour for those who run from the command line,
with --management-query-passwords, but still expect the prompt
on the console.

Note that the management interface will prompt for both username and
password ignoring the username read from the file. As most GUIs can
save the the username, this is a one-time inconvenience.

Currently, the password is queried on the console (or systemd)
in such cases. This is not sensible when console is not available
(windows GUI, tunnelblick etc.) or when the log is redirected
to a file on Windows (for some reason prompt goes to the log file).

Trac # 757

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1585591527-23734-2-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19655.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>