desk.cpl: Minor modifications for InitBackgroundDialog

In InitBackgroundDialog in background.c, do the following:

• Set the default value for the placement combobox before the registry is accessed
• Load the monitor bitmap before the registry is accessed
• Lower the requested access rights for RegOpenKeyEx to HKEY_QUERY_VALUE
• In case RegOpenKeyEx fails, simply return to the calling function
• Remove the variable varType since it is not needed
• Remove the variable result. It is only used to check if the registry functions have failed, so
  the function result can be used directly to do this.