Don't use environment variables to pass data from native to managed code

[davidfowl]

It flows to child contexts and generally messes things up.

[muratg]

@moozzyk

[blowdart]

Bonus potential security issues here as well :)

[moozzyk]

Fixing AppDomainManager may be interesting

[moozzyk]

When doing this we should also pass OS, Architecture etc. from the native layer to managed code.

[analogrelay]

Also the framework name selected using the --framework switch specified in #2333. Right now I'm going to be repurposing the DNX_FRAMEWORK environment variable to do this.

This one might be tricky since it needs to get to the DomainManager. Possibly a P/Invoke callback could be used?

[davidfowl]

Related to #2544. When you make this change, can you make it so that https://github.com/aspnet/dnx/blob/dev/src/Microsoft.Dnx.Host/Bootstrapper.cs has a richer object model that represents the things it takes as input? Basically a higher level hosting API that doesn't assume the caller is a command line application?