Xwayland non-standard modes follow-up

The current code, as changed by commit ad2d461d „Do not round non-standard modes“ is reported to be logically incongruent.

We should either drop libxcvt entirely or simply fix the size, keeping the CVT timings unchanged.

For backward compatibility and simplicity, I'd rather simply fix the hdisplay/vdisplay to match the given size.

Also let's fix the „modinfo“ spelling.

Closes: #1549 (closed)

/cc @libv

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