Fix memory corruption through InitString and second-engine loading

Fix memory corruption through InitString and second-engine loading

The InitString and ComputerString options were not swapped when replacing
the second engine, so that the ChessProgramStates of the first engine
could point to an invalid (already freed) init string. Also make sure
that appData.directory contains its value in allocated memory, even
when hand-loading engines.
  Some of the rare options were not swapped either, and would thus
always be loaded for the first engine when -singleEngineList was true.
They are now also all reset to their default, before engine load,
to prevent options of one engine to linger on for a next that does
not specify them. The intentional exception is -firstHost.