syslog_line_prefix

Here is a patch to add a GUC parameter "syslog_line_prefix".
It adds prefixes to syslog and eventlog. We still have
"log_line_prefix", that will be used only for stderr logs.

We have a tip that log_line_prefix is not required for syslog
in the documentation, but we'd better to have independent setttings
if we set log_destination to 'stderr, syslog'.

http://developer.postgresql.org/pgdocs/postgres/runtime-config-logging.html#RUNTIME-CONFIG-LOGGING-WHAT
| Tip: Syslog produces its own time stamp and process ID
| information, so you probably do not want to use those escapes
| if you are logging to syslog.

Regards,
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ITAGAKI Takahiro
NTT Open Source Software Center