ntlm: Fixed HTTP proxy authentication when using Windows SSPI

Removed ISC_REQ_* flags from calls to InitializeSecurityContext to fix
bug in NTLM handshake for HTTP proxy authentication.

NTLM handshake for HTTP proxy authentication failed with error
SEC_E_INVALID_TOKEN from InitializeSecurityContext for certain proxy
servers on generating the NTLM Type-3 message.

The flag ISC_REQ_CONFIDENTIALITY seems to cause the problem according
to the observations and suggestions made in a bug report for the
QT project (https://bugreports.qt-project.org/browse/QTBUG-17322).

Removing all the flags solved the problem.

Bug: http://curl.haxx.se/mail/lib-2014-08/0273.html
Reported-by: Ulrich Telle
Assisted-by: Steve Holme, Daniel Stenberg