Suppress timezone (%Z) part of timestamp display when running on Windows,

because on that platform strftime produces localized zone names in varying
encodings.  Even though it's only in a comment, this can cause encoding
errors when reloading the dump script.  Per suggestion from Andreas
Seltenreich.  Also, suppress %Z on Windows in the %s escape of
log_line_prefix ... not sure why this one is different from the other two,
but it shouldn't be.