Bad bug in fopen() wrapper code

["Magnus Hagander" <mha(at)sollentuna(dot)net>]

There is a small bug in the fopen() wrapper code that was applied a
couple of weeks back for win32. It sets the wrong flags for files opened
in "append" mode. This makes the logfile writing not work - syslog.c
opens the logfile in append mode, but if the file does not already
exist, it will not be opened and an error is returned - causing the
postmaster to terminate.

This is pretty bad and pretty urgent - with this, systems installed by
the MSI installer simply *do not start*, because they are by default
configured to write logs to a file...

Attached patch sets the  O_CREAT option when appending to files.

//Ma <<open.diff>> gnus

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