Add TLS and connectionless LDAP support to Linux

This commit manually specifies the LDAP URI option during connect
(but before binding). This is necessary because in order to know the
correct scheme, we need access to SessionOptions, which is not available
until after initialization.

Finally, it removes the PlatformUnsupportedException from the
SessionOptions.SecureSocketLayer property.

This makes it possible to use LDAP over TLS and connectionless (UDP)
LDAP.

Author: Isaiah Inuwa

src/libraries/System.DirectoryServices.Protocols/src/System/DirectoryServices/Protocols/ldap/LdapConnection.Linux.cs

@@ -32,8 +32,20 @@ private int InternalConnectToServer()
             // available during init.
             Debug.Assert(!_ldapHandle.IsInvalid);
 
-            string scheme = "ldap://";
+            string scheme = null;
             LdapDirectoryIdentifier directoryIdentifier = (LdapDirectoryIdentifier)_directoryIdentifier;
+            if (directoryIdentifier.Connectionless)
+            {
+                scheme = "cldap://";
+            }
+            else if (SessionOptions.SecureSocketLayer)
+            {
+                scheme = "ldaps://";
+            }
+            else
+            {
+                scheme = "ldap://";
+            }
 
             string uris = null;
             string[] servers = directoryIdentifier.Servers;

src/libraries/System.DirectoryServices.Protocols/src/System/DirectoryServices/Protocols/ldap/LdapSessionOptions.Linux.cs

@@ -9,12 +9,7 @@ public partial class LdapSessionOptions
     {
         private static void PALCertFreeCRLContext(IntPtr certPtr) { /* No op */ }
 
-        [SupportedOSPlatform("windows")]
-        public bool SecureSocketLayer
-        {
-            get => throw new PlatformNotSupportedException();
-            set => throw new PlatformNotSupportedException();
-        }
+        public bool SecureSocketLayer { get; set; }
 
         public int ProtocolVersion
         {