ntlm: fix negotiated flags usage

ntlm: fix negotiated flags usage

According to Microsoft document MS-NLMP, current flags usage is not
accurate: flag NTLMFLAG_NEGOTIATE_NTLM2_KEY controls the use of
extended security in an NTLM authentication message and NTLM version 2
cannot be negotiated within the protocol.

The solution implemented here is: if the extended security flag is set,
prefer using NTLM version 2 (as a server featuring extended security
should also support version 2). If version 2 has been disabled at
compile time, use extended security.

Tests involving NTLM are adjusted to this new behavior.

Fixes #6813
Closes #6849