`--unsafe' switch to enable access to Lua's global builtins

Currently we can't access the normal Lua global namespace from the configuration file. That's because 'ldoc' is set as its environment.

I guess it's a feature that's supposed to make running ldoc by automated tools, on internet servers, a "safe" thing. I.e., users aren't able to access "dangerous" functions like os.execute() or io.open().

However, this limitation is a great nuisance. So, this patch introduces an '--unsafe' switch that turns off that "feature".

(Or we can do the opposite: we can reverse the logic in that patch, make "unsafe" the default, and name the switch '--safe' or ('--sandboxed') instead.)