Create a built-in log rotation program, so that we no longer have to

Create a built-in log rotation program, so that we no longer have to
recommend that people go get Apache's rotatelogs program.  Additional
benefits are that configuration is done through GUC, rather than
externally, and that the postmaster can monitor the log rotator and
restart it after failure (though we certainly hope that won't happen
often).
Andreas Pflug, some rework by Tom Lane.