Support NegotiateStream after it lights up in dotnet/corefx

[iamjasonp]

Currently, System.Net.Security.NegotiateStream is stubbed out but not yet implemented.

• Windows implementation - dotnet/corefx#1844
• Linux implementation - dotnet/corefx#2483

[iamjasonp]

dotnet/corefx#5156 is the implementation port

[iamjasonp]

dotnet/corefx#5156 was merged, and packages should be published. I'm updating packages locally on my box to test this.

[iamjasonp]

To do:

• Update project.json files to package version rc2-23712
• ~~~Update project.lock.json (they're to be deprecated, but while they exist we still have to update them)~~~
• Update the build scripts to have SUPPORTS_WINDOWSIDENTITY be defined
• Allow TransportSecurity tests with NegotiateStream to run

(Currently, build.cmd targets OSGroup = AnyOS, which prevents SUPPORTS_WINDOWSIDENTITY from being defined) EDIT: fixed, see #695

[iamjasonp]

We're lit up in Windows! We are waiting for this to be available on Linux.

[roncain]

Added label to indicate we are blocked in the Linux implementation by https://github.com/dotnet/corefx/issues/2483