Implement Get-Process -IncludeUserName for Linux

So much plumbing. The correct way to identify the owner of a process on
Linux is to get the owner of the /proc/<PID> file. However, .NET Core
has no API to get this information, thus it had to be plumbed through to
the native library.

src/Microsoft.PowerShell.Commands.Management/commands/management/Process.cs

@@ -866,6 +866,9 @@ namespace Microsoft.PowerShell.Commands
         private static string RetrieveProcessUserName(Process process, Cmdlet cmdlet)
         {
             string userName = null;
+#if LINUX
+            userName = Platform.NonWindowsGetFileOwner("/proc/" + process.Id);
+#else
             IntPtr tokenUserInfo = IntPtr.Zero;
             IntPtr processTokenHandler = IntPtr.Zero;
 
@@ -955,6 +958,7 @@ namespace Microsoft.PowerShell.Commands
                 }
             }
 
+#endif
             return userName;
         }
 

src/System.Management.Automation/CoreCLR/CorePsPlatform.cs

@@ -354,6 +354,18 @@ namespace System.Management.Automation
             }
         }
 
+        internal static string NonWindowsGetFileOwner(string path)
+        {
+            if (!IsWindows)
+            {
+                return LinuxPlatform.GetFileOwner(path);
+            }
+            else
+            {
+                throw new PlatformNotSupportedException();
+            }
+        }
+
 #if CORECLR
         internal static string NonWindowsGetFolderPath(SpecialFolder folder)
         {
@@ -740,6 +752,11 @@ namespace System.Management.Automation
             return Native.FollowSymLink(path);
         }
 
+        public static string GetFileOwner(string path)
+        {
+            return Native.GetFileOwner(path);
+        }
+
         [StructLayout(LayoutKind.Sequential)]
         internal class SetDateInfoInternal
         {
@@ -809,6 +826,9 @@ namespace System.Management.Automation
             [return: MarshalAs(UnmanagedType.LPStr)]
             internal static extern string FollowSymLink([MarshalAs(UnmanagedType.LPStr)]string filePath);
 
+            [DllImport(psLib, CharSet = CharSet.Ansi, SetLastError = true)]
+            [return: MarshalAs(UnmanagedType.LPStr)]
+            internal static extern string GetFileOwner([MarshalAs(UnmanagedType.LPStr)]string filePath);
         }
     }