Make VACUUM VERBOSE report the number of skipped frozen pages.

Previously manual VACUUM did not report the number of skipped frozen
pages even when VERBOSE option is specified. But this information is
helpful to monitor the VACUUM activity, and also autovacuum reports that
number in the log file when the condition of log_autovacuum_min_duration
is met.

This commit changes VACUUM VERBOSE so that it reports the number
of frozen pages that it skips.

Author: Masahiko Sawada
Reviewed-by: Yugo Nagata and Jim Nasby

Discussion: http://postgr.es/m/CAD21AoDZQKCxo0L39Mrq08cONNkXQKXuh=2DP1Q8ebmt35SoaA@mail.gmail.com

doc/src/sgml/ref/vacuum.sgml

@@ -273,7 +273,7 @@ DETAIL:  CPU: user: 0.06 s, system: 0.01 s, elapsed: 0.07 s.
 INFO:  "onek": found 3000 removable, 1000 nonremovable tuples in 143 pages
 DETAIL:  0 dead tuples cannot be removed yet.
 There were 0 unused item pointers.
-Skipped 0 pages due to buffer pins.
+Skipped 0 pages due to buffer pins, 0 frozen pages.
 0 pages are entirely empty.
 CPU: user: 0.39 s, system: 0.07 s, elapsed: 1.56 s.
 INFO:  analyzing "public.onek"

src/backend/commands/vacuumlazy.c

@@ -1341,10 +1341,14 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
 					 nkeep, OldestXmin);
 	appendStringInfo(&buf, _("There were %.0f unused item pointers.\n"),
 					 nunused);
-	appendStringInfo(&buf, ngettext("Skipped %u page due to buffer pins.\n",
-									"Skipped %u pages due to buffer pins.\n",
+	appendStringInfo(&buf, ngettext("Skipped %u page due to buffer pins, ",
+									"Skipped %u pages due to buffer pins, ",
 									vacrelstats->pinskipped_pages),
 					 vacrelstats->pinskipped_pages);
+	appendStringInfo(&buf, ngettext("%u frozen page.\n",
+									"%u frozen pages.\n",
+									vacrelstats->frozenskipped_pages),
+					 vacrelstats->frozenskipped_pages);
 	appendStringInfo(&buf, ngettext("%u page is entirely empty.\n",
 									"%u pages are entirely empty.\n",
 									empty_pages),