Overhaul time support API

This patch changes the duration_* API in several ways.  First, it
updates the API to use better names.  Second, string formatting has
been removed from the API (with its associated malloc).  Finally, a
new function added to convert the time into seconds, which can be
used (or formatted) by the caller.  This eliminates hidden calls to
malloc that require associated calls to free().

This patch also removes the useless extern keyword from prototypes,
and it eliminates the duration_t typedef (use 'struct duration').
These API also allows proper error checking, as it is possible for
gettimeofday to fail in certain circumstances.

The consumers have all been chased to use this new API as well, as
there were relatively few cases doing this type of measurement.
In most cases, the code performs additional checks for errors, but
the calling code looks much cleaner in every case.

src/helper/time_support.c

@@ -28,7 +28,6 @@
 #endif
 
 #include "time_support.h"
-#include "log.h"
 
 
 /* calculate difference between two struct timeval values */
@@ -83,29 +82,30 @@ int timeval_add_time(struct timeval *result, int sec, int usec)
 	return 0;
 }
 
-void duration_start_measure(duration_t *duration)
+int duration_start(struct duration *duration)
 {
-	gettimeofday(&duration->start, NULL);
+	return gettimeofday(&duration->start, NULL);
 }
 
-int duration_stop_measure(duration_t *duration, char **text)
+int duration_measure(struct duration *duration)
 {
 	struct timeval end;
+	int retval = gettimeofday(&end, NULL);
+	if (0 == retval)
+		timeval_subtract(&duration->elapsed, &end, &duration->start);
+	return retval;
+}
 
-	gettimeofday(&end, NULL);
+float duration_elapsed(struct duration *duration)
+{
+	float t = duration->elapsed.tv_sec;
+	t += (float)duration->elapsed.tv_usec / 1000000.0;
+	return t;
+}
 
-	timeval_subtract(&duration->duration, &end, &duration->start);
-
-	if (text)
-	{
-		float t;
-		t = duration->duration.tv_sec;
-		t += (float)duration->duration.tv_usec/1000000.0;
-		*text = malloc(100);
-		snprintf(*text, 100, "%fs", t);
-	}
-
-	return ERROR_OK;
+float duration_kbps(struct duration *duration, size_t count)
+{
+	return count / (1024.0 * duration_elapsed(duration));
 }
 
 long long timeval_ms()