Bug with prolog_style_variables

[mccune]

A user just pointed out that weighting rules containing the special symbol "_" (which matches any variable and only a variable) do not work
correctly when prolog_style_variables is set (because "_" is a prolog_style_variable). The proposed fix is to change the definition of
prolog_style_variables so that it no longer includes symbols that start with "_".

Any objections?

Bill