pgsql: Fix problem with infinite recursion between write_syslogger_file

Log Message:
-----------
Fix problem with infinite recursion between write_syslogger_file and
elog if the former has trouble writing its file. Code review for
Magnus' patch to redirect stderr to syslog on Windows (Bruce's version
seems right, but did some minor prettification).

Backpatch both changes to 8.0 branch.

Modified Files:
--------------
pgsql/src/backend/postmaster:
syslogger.c (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c.diff?r1=1.13&r2=1.14)
pgsql/src/backend/utils/error:
elog.c (r1.157 -> r1.158)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.157&r2=1.158)