Add a summary table to the log that shows each drives status

Add a summary table to the log that shows each drives status
i.e. erased or failed, throughput, duration of wipe, model,
serial no etc. In particular it benefits those that wipe many
drives simultaneously in rack servers. At a glance any failed
drives can be seen without having to browse back through the
log. Especially useful in --nogui mode, but also useful in GUI
mode.

src/context.h

@@ -120,6 +120,9 @@ typedef struct nwipe_context_t_
     int wipe_status;  // Wipe finished = 0, wipe in progress = 1, wipe yet to start = -1.
     int spinner_idx;  // Index into the spinner character array
     char spinner_character[1];  // The current spinner character
+    double duration;  // Duration of the wipe in seconds
+    time_t start_time;  // Start time of wipe
+    time_t end_time;  // End time of wipe
     /*
      * Identity contains the raw serial number of the drive
      * (where applicable), however, for use within nwipe use the