control: Dump flight recorder to a JSON file

A future commit will dump more debug data as opposed to just the flight
recorder.  To better support the data all being in the same file, it
will be a JSON file.  The first step of that is to write the flight
recorder output to a JSON file.

This also reorganizes the Manager code that does it to prepare for
different data also being in the same file.

An example is:
{
    "flight_recorder": [
        "Oct 06 05:59:01.183998: main: Startup"
    ]
}

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Iaeb55ffde3a30c2345968e1b3fad313b50aff331
diff --git a/control/fanctl.cpp b/control/fanctl.cpp
index 6df14f0..6822b5d 100644
--- a/control/fanctl.cpp
+++ b/control/fanctl.cpp
@@ -531,7 +531,7 @@
     {
         SDBusPlus::callMethod(systemdService, systemdPath, systemdMgrIface,
                               "KillUnit", phosphorServiceName, "main", SIGUSR1);
-        std::cout << "FlightRecorder log written to: /tmp/fan_control.txt"
+        std::cout << "FlightRecorder log written to: /tmp/fan_control_dump.json"
                   << std::endl;
     }
     catch (const phosphor::fan::util::DBusPropertyError& e)