Porting/bench.pl: new syntax for perl args and env

Porting/bench.pl: new syntax for perl args and env

The existing syntax for supplying per-perl-executable command-line
arguments and environment was clunky and buggy.

Replace it with the following syntax and add tests:

One of more perls to execute is listed on the command line after all
switches have been processed. Each perl is of the form:

    some-perl [--args='-Mstrict -Mwarnings' ] \
              [ --env='A=a' --env='B=b' .... ]

i.e. the name of a perl executable, optionally succeeded by --args and
--env switches.

E.g. the above invokes

    'some-perl -Mstrict -Mwarnings'

with the A and B environment variables set.

See http://nntp.perl.org/group/perl.perl5.porters/240004