Allow one to specify a DefaultHGRegion flag in [GridService] in order to allow different default regions for HG and direct grid logins.

This requires a new GridService.GetDefaultHypergridRegions() so ROBUST services require updating but not simulators.
This method still returns regions flagged with just DefaultRegion after any DefaultHGRegions, so if no DefaultHGRegions are specified
then existing configured defaults will still work.
Immediate use is for conference where we need to be able to specify different defaults
However, this is also generally useful to send experienced HG users to one default location and local users whose specified region fails (e.g. no "home" or "last") to another.

OpenSim/Data/MySQL/MySQLRegionData.cs

@@ -310,6 +310,11 @@ namespace OpenSim.Data.MySQL
             return Get((int)RegionFlags.DefaultRegion, scopeID);
         }
 
+        public List<RegionData> GetDefaultHypergridRegions(UUID scopeID)
+        {
+            return Get((int)RegionFlags.DefaultHGRegion, scopeID);
+        }
+
         public List<RegionData> GetFallbackRegions(UUID scopeID, int x, int y)
         {
             List<RegionData> regions = Get((int)RegionFlags.FallbackRegion, scopeID);