pgbench: fix stats reporting when some transactions are skipped.

pgbench can skip some transactions when both -R and -L options are used.
Previously, this resulted in slightly silly statistics both in progress
reports and final output, because the skipped transactions were counted
as executed for TPS and related stats.  Discount skipped xacts in TPS
numbers, and also when figuring the percentage of xacts exceeding the
latency limit.

Also, don't print per-script skipped-transaction counts when there is
only one script.  That's redundant with the overall count, and it's
inconsistent with the fact that we don't print other per-script stats
when there's only one script.  Clean up some unnecessary interactions
between what should be independent options that were due to that
decision.

While at it, avoid division-by-zero in cases where no transactions were
executed.  While on modern platforms this would generally result in
printing "NaN" rather than a crash, that isn't spelled consistently
across platforms and it would confuse many people.  Skip the relevant
output entirely when practical, else print zeroes.

Fabien Coelho, reviewed by Steve Singer, additional hacking by me

Discussion: https://postgr.es/m/26654.1505232433@sss.pgh.pa.us

doc/src/sgml/ref/pgbench.sgml

@@ -1363,8 +1363,7 @@ latency average = 15.844 ms
 latency stddev = 2.715 ms
 tps = 618.764555 (including connections establishing)
 tps = 622.977698 (excluding connections establishing)
-script statistics:
- - statement latencies in milliseconds:
+statement latencies in milliseconds:
         0.002  \set aid random(1, 100000 * :scale)
         0.005  \set bid random(1, 1 * :scale)
         0.002  \set tid random(1, 10 * :scale)