If I'm correct, this makes the complicated evaluator code a little simpler without introducing a regression.
The "if (tailcall <= 0)" block is redundant with what's in lambda_qm, which is what
gets executed if no error are encountered in the meantime.
I'm less confident about removing "tailcall=0", but it could be that this was only
done when the code called llocal() instead of add_variables_to_var_stack.
llocal() is a no-op when tailcall==0, but add_variables_to_var_stack executes
regardless of the value of tailcall.
