Fix NegotitateStream (server) on Linux for NTLM (dotnet/corefx#42522)

Fix NegotitateStream (server) on Linux for NTLM (dotnet/corefx#42522)

This PR is a follow up to PR dotnet/corefx#36827 which added support for Linux server-side
GSS-API (AcceptSecContext). This enabled NegotitateStream AuthenticateAsServer*
support. It also provided support for ASP.NET Core to allow Kestrel server to have
Negotiate authentication on Linux.

This PR fixes some problems with Negotiate (SPNEGO) fallback from Kerberos to NTLM.
Notably it passes in a correct GSS Acceptor credential so that fallback will work
correctly. As part of fixing that, I noticed some other problems with returning the
user-identity when NTLM is used.

This was tested in a separate enterprise testing environment that I have created.
It builds on technologies that we have started using like docker containers and Azure
pipelines (e.g. HttpStress). The environment is currently here:
https://dev.azure.com/systemnetncl/Enterprise%20Testing. The extra Kerberos tests
and container support is here: https://github.com/davidsh/networkingtests

When the repo merge is completed, I will work with the infra team to see what things
can be merged back into the main repo/CI pipeline and migrate the test sources to an
appropriate place in the new repo.

Contributes to dotnet/corefx#10041
Contributes to dotnet/corefx#24707
Contributes to dotnet/corefx#30150

Commit migrated from https://github.com/dotnet/corefx/commit/1054f1fba1b7222c138dcd20d8da80cd893fdd01