pgsql: Only use the pipe chunking protocol if we know the syslogger

Log Message:
-----------
Only use the pipe chunking protocol if we know the syslogger should
be catching stderr output, and we are not ourselves the
syslogger. Otherwise, go directly to stderr.
Bug noticed by Tom Lane.
Backpatch as far as 8.0.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        postmaster.c (r1.532 -> r1.533)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.532&r2=1.533)
        syslogger.c (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c.diff?r1=1.32&r2=1.33)
    pgsql/src/backend/utils/error:
        elog.c (r1.187 -> r1.188)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.187&r2=1.188)