SysLogger subprocess

Here's the core of the logger subprocess.

- no rotation code so far, all syslogFile handling is for testing only
- send_message_to_serverlog needs some careful handling of stderr, in
case pipe writing fails or if after a log process restart redirecting
stderr fails. In these cases, the original stderr should be used.

While looking at pgstat.c to see how to peek for pipe data, I found
readpipe=pgStatPipe[0];
select(readPipe+1, ..);

which is probably usually the same as select(pgStatPipe[1], ..) This fd
arithmetics seem a bit dubious to me.

Regards,
Andreas