Don't allow remote mythbackends to upgrade database.

Only allow the mythbackend application running on the master backend
host to upgrade the database.  Upgrades can still be performed by
mythtv-setup on any host (including master backend host, remote backend
hosts, and dedicated mythfrontend hosts).

(cherry picked from commit 889f533)

mythtv/programs/mythbackend/main_helpers.cpp

@@ -660,16 +660,16 @@ int run_backend(const MythCommandLineParser &cmdline)
     if (!setup_context(cmdline))
         return BACKEND_EXIT_NO_MYTHCONTEXT;
 
-    if (!UpgradeTVDatabaseSchema(true, true))
+    bool ismaster = gCoreContext->IsMasterHost();
+
+    if (!UpgradeTVDatabaseSchema(ismaster, ismaster))
     {
         VERBOSE(VB_IMPORTANT, "Couldn't upgrade database to new schema");
         return BACKEND_EXIT_DB_OUTOFDATE;
     }
 
     ///////////////////////////////////////////
 
-    bool ismaster = gCoreContext->IsMasterHost();
-
     g_pUPnp = new MediaServer(ismaster, !cmdline.IsUPnPEnabled() );
 
     if (!ismaster)