pgbench: Support Multiple Simultaneous Runs (with Mean and Std. Dev.)

I created a version of this a while back for a comparison of 7.4.x to
8.0.x. Now that 8.1.x has come out, I've decided to go ahead and put
it in patch form for potential inclusion in contrib.

The main addition is the addition of a -x option that allows
specification of a number of successive runs of pgbench for use in
sanity-checking basic benchmark results to reduce the potential for
noise in a single run.

At the end of a series of runs, the mean tps and standard deviations
are reported.

I only patched the English README.

I didn't test compatibility with the new file code, but a cursory
glance at the code makes me think it would work.

The following patch is against a fresh pull from anonymous CVS.