Fixes #10153. Guess at DVB-S vs DVB-S2 when not explicitly set.

The problem was discovered by Lawrence Rust. It has been decided not to fix the DVB API regression upstream, so this code attempts to guess which system is being used using the modulation scheme as a hint.

mythtv/libs/libmythtv/dtvmultiplex.cpp

@@ -262,6 +262,22 @@ bool DTVMultiplex::ParseDVB_S2(
         return false;
     }
 
+    // For #10153, guess at modulation system based on modulation
+    if (DTVModulationSystem::kModulationSystem_UNDEFINED == mod_sys)
+    {
+        mod_sys = (DTVModulation::kModulationQPSK == modulation) ?
+            DTVModulationSystem::kModulationSystem_DVBS : 
+            DTVModulationSystem::kModulationSystem_DVBS2;
+    }
+
+    if ((DTVModulationSystem::kModulationSystem_DVBS  != mod_sys) &&
+        (DTVModulationSystem::kModulationSystem_DVBS2 != mod_sys))
+    {
+        LOG(VB_GENERAL, LOG_ERR, LOC + "Unsupported S2 modulation system " +
+            QString("parameter '%1', aborting.").arg(_mod_sys));
+        return false;
+    }
+
     if (!_rolloff.isEmpty())
         ok &= rolloff.Parse(_rolloff);