write_pipe_chunks patch messes up early error message output

[ ok, let's try this again --- apparently there's something in my mail
software that dislikes embedded nulls ]

I was experimenting with what happened if I made the pg_log directory
unwritable, and found out that this comes out on stderr:

<NUL><NUL><NUL>_<NUL><NUL>J3tFATAL: could not create log file "pg_log/postgresql-2007-07-15_210716.log": Permission denied

The problem is that elog.c thinks that the GUC variable Redirect_stderr
tells it whether to use the pipe chunk protocol on stderr, but in fact
that should *not* happen until we have successfully launched the syslogger
and repointed stderr to the pipe.

I suspect that error messages coming out of the syslogger itself (and
directed to the original stderr destination) will be similarly broken.

So that patch still needs work. I think we probably need a flag
variable separate from the GUC variable to tell when to send using
the chunk protocol.

regards, tom lane