Suppress time zone name (%Z) when logging timestamps in xlog.c startup

Suppress time zone name (%Z) when logging timestamps in xlog.c startup
on Windows.  This is yet another manifestation of the problem that Windows
returns time zone names that may be in a different encoding than we are using.
I've put a better solution in HEAD, but the back branches need a simple patch.
Per report from Hiroshi Saito.