Make skipping of unknown option smarter

Make skipping of unknown option smarter

To make XBoard better resistant to the presence of disbanded or future
options in its settings file, encountering those is no longer a fatal error.
But the error recovery only consisted of skipping the rest of the line.
This fails badly on string options with a multi-line value, which is
enclosed by {braces}. We now detect that case, to skip to } instead.