pgsql: Fix syslog bug: if any messages are emitted to write_syslog

Log Message:
-----------
Fix syslog bug: if any messages are emitted to write_syslog before
the facility has been set, the facility gets set to LOCAL0 and cannot
be changed later. This seems reasonably plausible to happen, particularly
at higher debug log levels, though I am not certain it explains Han Holl's
recent report. Easiest fix is to teach the code how to change the value
on-the-fly, which is nicer anyway. I made the settings PGC_SIGHUP to
conform with log_destination.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.28&r2=1.29)
pgsql/src/backend/utils/error:
elog.c (r1.163 -> r1.164)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.163&r2=1.164)
pgsql/src/backend/utils/misc:
guc.c (r1.291 -> r1.292)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.291&r2=1.292)
pgsql/src/include/utils:
elog.h (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/elog.h.diff?r1=1.79&r2=1.80)