Limit access to non-shared mode

A read-only client should not be allowed to kick out other clients.
It will be forced into shared mode, or refused the connection, depending
on the neverShared parameter.
diff --git a/common/rfb/SConnection.cxx b/common/rfb/SConnection.cxx
index 20182a0..99a4850 100644
--- a/common/rfb/SConnection.cxx
+++ b/common/rfb/SConnection.cxx
@@ -42,6 +42,7 @@
 const SConnection::AccessRights SConnection::AccessPtrEvents      = 0x0004;
 const SConnection::AccessRights SConnection::AccessCutText        = 0x0008;
 const SConnection::AccessRights SConnection::AccessSetDesktopSize = 0x0010;
+const SConnection::AccessRights SConnection::AccessNonShared      = 0x0020;
 const SConnection::AccessRights SConnection::AccessDefault        = 0x03ff;
 const SConnection::AccessRights SConnection::AccessNoQuery        = 0x0400;
 const SConnection::AccessRights SConnection::AccessFull           = 0xffff;