This allows a nodedef to specify a fixed value for param2 to be
used for all normal placements.
There are several uses for this:
- nodes that require param2 to be set to a non-zero value for
internal mod use. E.g. leafdecay could use this to detect that
leaves are played by players.
- force wallmounted or facedir value at placement at placement
This overrides any player look direction or other on-the-fly
param2 setting during placement.
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