pgsql: On Windows, syslogger runs in two threads.

Log Message:
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On Windows, syslogger runs in two threads. The main thread processes config
reload and rotation signals, and a helper thread reads messages from the
pipe and writes them to the log file. However, server code isn't generally
thread-safe, so if both try to do e.g palloc()/pfree() at the same time,
bad things will happen. To fix that, use a critical section (which is like
a mutex) to enforce that only one the threads are active at a time.

Modified Files:
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pgsql/src/backend/postmaster:
syslogger.c (r1.56 -> r1.57)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c?r1=1.56&r2=1.57)