Replaced no longer functional __MonoCS__ flag with custom flag

`__MonoCS__` was always considered a hack, and with Mono's conversion to msbuild it's no longer defined.  Intsead of leaving the old semantic, I've instead declared a new flag, `_MONO_CLI_FLAG_`, that says where's it's from and what it means.

The AppVeyor build script likewise declares it, and anyone compiling with Mono should likewise declare it until such time as this Windows-specific code can be excised.

.appveyor.yml

@@ -52,7 +52,7 @@ for:
     only:
     - image: Ubuntu
   build_script:
-  - msbuild Halcyon.sln
+  - msbuild /p:DefineConstants="_MONO_CLI_FLAG_" Halcyon.sln
 
 after_build:
 - ps: |

OpenSim/Framework/Util.cs

@@ -53,7 +53,7 @@
 using System.Drawing;
 using System.Runtime.InteropServices;
 
-#if !__MonoCS__
+#if !_MONO_CLI_FLAG_
 using System.DirectoryServices.AccountManagement;
 #endif
 
@@ -186,7 +186,7 @@ public static UInt64 GetLongTickCount()
         /// <param name='password'>string</param>
         public static bool AuthenticateAsSystemUser(string username, string password)
         {
-            #if __MonoCS__
+            #if _MONO_CLI_FLAG_
                 // TODO: find a way to check the user info cross platform.  In the mean time better security by NOT allowing remote admin.
                 return false;
             #else