Implement saving of (modified) engine settings

Implement saving of (modified) engine settings

The Engine #N Settings dialogs will now have an extra button at the top,
labelled "Make Persistent". Pressing it will 'OK' the dialog, and in
addition will add all non-default settings of the options to the engine
line in the Engine List (-firstChessProgramNames), as a -firstOptions
XBoard option. If such an option is already in the engine line, its value
will be altered to the newly specified settings. The function ResendOptions
was modified to also allow returning the options as a string, rather
than sending them as 'option' commands to the engine.
 Currently this will only work for engines that were loaded through the
Load Engine dialogs, not for those specified with -fcp/scp at startup.
Because the latter need not be in the engine list. The Load Engine
dialogs will make XBoard remember the entire engine line of the engine
that was last loaded, and pressing the button will search a line exactly
like it in the engine list, to add/change its -firstOptions option.
It would fail silently if no such line is found.