Create MS.SqlTools.Credentials project (#249)

* Initial credential service files

* Clean-up hostloader

* Update build scripts to build credentials archive

* Move hosting files to new assembly

* Add credentials files to MS.SqlTools.Credentials

* Remove duplicate files

* Update namespace in program.cs

* Fix test build breaks

* Update extensions visibility.

* Remove unused resource strings

* Add xproj files to SLN for appveyor builds

* Fix appveyor build break in test project

* Fix extensibility tests

* Fix various typos in latest iteration

* Add settings for Integration build

* Fix codecoverage.bat to use full pdb for new projects

* Fix bug when packing in folder with native images

* Fix typos in xproj

* Reset XLF to fix build.cmd

src/Microsoft.SqlTools.Credentials/Credentials/Linux/Interop.Sys.cs

@@ -0,0 +1,47 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using System.Runtime.InteropServices;
+
+namespace Microsoft.SqlTools.ServiceLayer.Credentials
+{
+
+#if !WINDOWS_ONLY_BUILD
+
+    internal static partial class Interop
+    {
+        internal static partial class Sys
+        {
+            [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_ChMod", SetLastError = true)]
+            internal static extern int ChMod(string path, int mode);
+            
+            internal struct Passwd
+            {
+                internal IntPtr Name;           // char*
+                internal IntPtr Password;       // char*
+                internal uint  UserId;
+                internal uint  GroupId;
+                internal IntPtr UserInfo;       // char*
+                internal IntPtr HomeDirectory;  // char*
+                internal IntPtr Shell;          // char*
+            };
+
+            [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_GetPwUidR", SetLastError = false)]
+            internal static extern int GetPwUidR(uint uid, out Passwd pwd, IntPtr buf, int bufLen);
+
+            [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_GetEUid")]
+            internal static extern uint GetEUid();
+
+            private static partial class Libraries
+            {
+                internal const string SystemNative = "System.Native";
+            }
+        }
+    }
+
+#endif
+
+}