Duke3D: improve how labels are loaded into memory and introduce MAXLABELS limit

Currently, the game hijacks the sprite, wall and sector arrays to store the label data temporarily, which then gets copied into freshly allocated arrays once the scripts are fully parsed.

This leads to the maximum number of labels depending on MAXSPRITES, which is undesirable as the resulting value is rather arbitrary (11264). With this merge request, this coupling is removed, and an independent limit MAXLABELS would be introduced, which is currently set to 16384.

Additionally, with this merge, the limit is checked before character data is loaded into memory, rather than after everything has been loaded. Before, the game could potentially just happily write outside the array boundaries.