As discussed in GitHub issue #624 Lua scripts should not use
variables that are never going to be read, because the memory
for variable names is never going to be freed.
Add an optional `ifexist` parameter to the `set_var` function
that allows a Lua developer to set variables that are going to
be ignored if the variable name was not used elsewhere before.
Usually this mean that there is no `var()` sample fetch for the
variable in question within the configuration.
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