Fix a crash due to brittle va_lists on x86_64

The problem is that va_lists are handled in a brittle way by x86_64 and
need to be copied whenever even a small breeze blows. More info:-
http://www.bailopan.net/blog/?p=30

More information on this patch is here :-
http://article.gmane.org/gmane.comp.web.polipo.user/2931

The patch was originally against Polipo-1.0.4.1 but has been modified
to fit in with the latest master branch.