Remember log messages that happen before logs are configured

(And replay them once we know our first real logs.)

This is an implementation for issue 6938.  It solves the problem of
early log mesages not getting sent to log files, but not the issue of
early log messages not getting sent to controllers.

changes/ticket6938

@@ -0,0 +1,4 @@
+  o Minor features:
+
+    - When opening a log file at startup, send it every log message that we
+      generated between startup and opening it.  Closes ticket 6938.