Porting Enable-PSRemoting and its Tests (#2671)

Fixes #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.

build.psm1

@@ -252,7 +252,8 @@ cmd.exe /C cd /d "$location" "&" "$($vcPath)\vcvarsall.bat" "$Arch" "&" cmake "$
         $FilesToCopy = @('pwrshplugin.dll', 'pwrshplugin.pdb')
         $dstPath = "$PSScriptRoot\src\powershell-win-core"
         $FilesToCopy | ForEach-Object {
-            $srcPath = Join-Path (Join-Path (Join-Path (Get-Location) "bin") $Configuration) "CoreClr/$_"
+            $srcPath = [IO.Path]::Combine((Get-Location), "bin", $Configuration, "CoreClr/$_") 
+
             log "  Copying $srcPath to $dstPath"
             Copy-Item $srcPath $dstPath
         }

src/System.Management.Automation/engine/InitialSessionState.cs

@@ -5963,6 +5963,10 @@ internal static class PSSnapInHelpers
                 {"Debug-Job",                         new SessionStateCmdletEntry("Debug-Job", typeof(DebugJobCommand), helpFile) },
                 {"Disable-PSSessionConfiguration",    new SessionStateCmdletEntry("Disable-PSSessionConfiguration", typeof(DisablePSSessionConfigurationCommand), helpFile) },
                 {"Disconnect-PSSession",              new SessionStateCmdletEntry("Disconnect-PSSession", typeof(DisconnectPSSessionCommand), helpFile) },
+#if !UNIX
+                {"Disable-PSRemoting",                new SessionStateCmdletEntry("Disable-PSRemoting", typeof(DisablePSRemotingCommand), helpFile) },
+                {"Enable-PSRemoting",                 new SessionStateCmdletEntry("Enable-PSRemoting", typeof(EnablePSRemotingCommand), helpFile) },
+#endif
                 {"Enable-PSSessionConfiguration",     new SessionStateCmdletEntry("Enable-PSSessionConfiguration", typeof(EnablePSSessionConfigurationCommand), helpFile) },
                 {"Enter-PSHostProcess",               new SessionStateCmdletEntry("Enter-PSHostProcess", typeof(EnterPSHostProcessCommand), helpFile) },
                 {"Enter-PSSession",                   new SessionStateCmdletEntry("Enter-PSSession", typeof(EnterPSSessionCommand), helpFile) },
@@ -6013,8 +6017,6 @@ internal static class PSSnapInHelpers
                 {"Where-Object",                      new SessionStateCmdletEntry("Where-Object", typeof(WhereObjectCommand), helpFile) },
 #if !CORECLR
                 {"Add-PSSnapin",                      new SessionStateCmdletEntry("Add-PSSnapin", typeof(AddPSSnapinCommand), helpFile) },
-                {"Disable-PSRemoting",                new SessionStateCmdletEntry("Disable-PSRemoting", typeof(DisablePSRemotingCommand), helpFile) },
-                {"Enable-PSRemoting",                 new SessionStateCmdletEntry("Enable-PSRemoting", typeof(EnablePSRemotingCommand), helpFile) },
                 {"Export-Console",                    new SessionStateCmdletEntry("Export-Console", typeof(ExportConsoleCommand), helpFile) },
                 {"Get-PSSnapin",                      new SessionStateCmdletEntry("Get-PSSnapin", typeof(GetPSSnapinCommand), helpFile) },
                 {"Remove-PSSnapin",                   new SessionStateCmdletEntry("Remove-PSSnapin", typeof(RemovePSSnapinCommand), helpFile) },