[MERGE] test and fix up Porting/bench.pl

bench.pl didn't have  a proper test suite, and some command-line options
had suffered from bit rot. For example, --bisect no longer worked.
This branch adds such a test suite, and includes various fix ups and
improvements. In particular:

* Conceptually reorganise bench.pl into input, benchmark, output:
   1) read in zero or more previous runs;
   2) possibly do another run, and aggregate results with any reads;
   3) output the aggregate of (1) and (2) in various ways. There are three
      possible outputs: save raw data for later with --write, display in
      human-readable form with --show, and alter exit value with --bisect.

   bench.pl kind of did all that anyway, but with that model, I've made it
   allow --bisect with --show or --read for example. There's no reason
   why you can't do all three of: --write the results, --show the results,
   and alter the exit value with --bisect.
   Options like --norm, --compact etc are now conceptually sub-options of
   --show.
   The pod has been re-organised to reflect this.

* add -autolabel option, which generates unique labels for those perls
  which haven't got one.

* new syntax for per-perl args and environment:
    ./perl --args=..., --env=....

* fix broken --bisect; it also now displays the field value on stdout.

* Allow 1 only perl under --raw

* better error checks

* General code cleanup

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