gcc 4.6 warnings -Wunused-but-set-variable

As you might have heard, GCC 4.6 was released the other day.  It
generates a bunch of new warnings with the PostgreSQL source code, most
of which belong to the new warning scenario -Wunused-but-set-variable,
which is included in -Wall.

Attached is a patch that gets rid of most of these.  As you can see,
most of these remove real leftover garbage.  The line I marked in
pg_basebackup.c might be an actual problem: It goes through a whole lot
to figure out the timeline and then doesn't do anything with it.  In
some other cases, however, one might argue that the changes lose some
clarity, such as when dropping the return value of strtoul() or
va_arg().  How should we proceed?  In any case, my patch should be
re-reviewed for any possible side effects that I might have hastily
removed.