Fixes PowerShell#1193 for most scenarios. The remaining scenario to be addressed is the Nano Server bring-up scenario. To continue supporting that scenario, I left the Install-PowerShellRemoting script in place.

This change 
1. Ports Enable-PSRemoting and Disable-PSRemoting to PowerShell Core
2. Adds side-by-side PowerShell Core remoting support to the PSSessionConfiguration cmdlets and PSRemoting cmdlets.
3. Ports PSSessionConfiguration tests

This change also introduces a behavioral difference. The PSRemoting and PSSessionConfiguration cmdlets are now context-sensitive and only work for endpoints that match the PowerShell type. For example, Get-PSSessionConfiguration, when running in PowerShell Core, will only return PowerShell Core WinRM endpoints. It will only modify PowerShell Core WinRM endpoints and cannot be used to configure Windows PowerShell endpoints.