The intuitive behavior of des.levregion or des.teleport_region when
"exclude" is left unspecified is that there is no exclusion area.
However, this wasn't actually the case: since l_get_lregion defaulted
the exclusion area to (0,0,0,0) and exclude_islev to 0, this meant that
the 0,0 space on the map would always be excluded from regions. In cases
where a region was specified with its inclusion area constrained to the
0,0 space of the map, this would create a "Couldn't place lregion"
impossible message.
This fixes that issue by defaulting the exclusion area to (-1,-1,-1,-1),
and if the exclusion area is left unspecified, forces exclude_islev=1.
This means that the exclusion zone will be outside the walkable space of
the level where it can't cause any problems.
If a level designer puts negative coordinates in their inclusion or
exclusion parameters, this might not work correctly, but negative region
coordinates aren't currently used anywhere and probably shouldn't be
supported anyway.
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