Add OnMapInit forward and deprecate OnLevelInit

The change in behaviour to the OnLevelInit forward params isn't obvious
when compiling a plugin, deprecate it to make it a lot more obvious that
something has changed.

Some plugins rely just on the timing of OnLevelInit rather than doing
anything with the entity lump, for these plugins offer a new OnMapInit
forward that is implemented in core rather than sdkhooks. If / when we
offer a new entity lump manipulation API in the future this'll be the
forward where it can be used to make changes.

plugins/include/sdkhooks.inc

@@ -382,6 +382,7 @@ forward Action OnGetGameDescription(char gameDesc[64]);
  * @param mapEntities   Unused, always empty
  * @return              Unused, return value is ignored
  */
+#pragma deprecated Use OnMapInit() instead
 forward Action OnLevelInit(const char[] mapName, char mapEntities[2097152]);
 
 /**

plugins/include/sourcemod.inc

@@ -182,10 +182,14 @@ forward void OnPluginPauseChange(bool pause);
 forward void OnGameFrame();
 
 /**
- * Called when the map is loaded.
+ * Called when the map starts loading.
  *
- * @note This used to be OnServerLoad(), which is now deprecated.
- *       Plugins still using the old forward will work.
+ * @param mapName Name of the map
+ */
+forward void OnMapInit(const char[] mapName);
+
+/**
+ * Called when the map is loaded.
  */
 forward void OnMapStart();