MEDIUM: lua: Add `ifexist` parameter to `set_var`

MEDIUM: lua: Add `ifexist` parameter to `set_var`

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.