Fixed inaccurate ETAs: use current average rate over last 30 seconds instead of global rate

using global rate gives wildly inacurrate ETAs if rate changes over time.

this makes pv keep a history of progress over the last 30s interval and compute
current average rate over that, which fixes the issue.

fixed -a, --average-rate: show current average rate instead of global rate

[ quick test ]

- in one shell:
    $ dd if=/dev/zero of=/tmp/foo bs=1M count=5                 # simulate fast initial rate,
    $ (while true; do ls /*; sleep 1; done) >> /tmp/foo         # then slow rate. leave running ...

- in another shell:
    $ tail -f -n +1 /tmp/foo | pv -s 800M > /dev/null

    5.62MiB 0:00:17 [33.1KiB/s] [>                              ]  0% ETA 0:40:02
        bad:  (pv 1.6.6 - 1.6.20)
        ETA of a few minutes that keeps increasing ever and ever ...

    5.62MiB 0:00:16 [33.1KiB/s] [>                              ]  0% ETA 6:45:53
        good:
        ETA of 6+ hours, which is how long it'll take to reach 800Mb at 33k/s

src/include/pv-internal.h

@@ -45,6 +45,12 @@ extern "C" {
 #define MAXIMISE_BUFFER_FILL	1
 
 
+typedef struct pvhistory {
+	long long   total_bytes;
+	long double elapsed_sec;
+} pvhistory_t;
+
+
 /*
  * Structure for holding PV internal state. Opaque outside the PV library.
  */
@@ -113,6 +119,15 @@ struct pvstate_s {
 	long double prev_elapsed_sec;
 	long double prev_rate;
 	long double prev_trans;
+
+	/* Keep track of progress over last intervals to compute current average rate. */
+	pvhistory_t *history;            /* state at previous intervals (circular buffer) */
+	int history_len;		 /* total size */
+	int history_interval;		 /* seconds between each history entry */
+	int history_first;
+	int history_last;
+	long double current_avg_rate;    /* current average rate over last history intervals */
+	
 	unsigned long long initial_offset;
 	char *display_buffer;
 	long display_buffer_size;