Commit ad5d46a4494b0b480a3af246bb4227d9bdadca37 thought that we could
get around the known portability issues of strftime's %Z specifier by
using %z instead. However, that idea seems to have been innocent of
any actual research, as it certainly missed the facts that
(1) %z is not portable to pre-C99 systems, and
(2) %z doesn't actually act differently from %Z on Windows anyway.
Per failures on buildfarm member hamerkop.
While at it, centralize the code defining what strftime format we
want to use in pg_dump; three copies of that string seems a bit much.
f455fcfdb8