remove environment variables related to ConEmu, as they are not passed to the child process

See for example:
http://www.cpantesters.org/cpan/report/8e743c30-6c36-1014-878a-6534a694d5e7

The issue seems to be with unexpected environment variables:
CONEMUARGS, CONEMUANSILOG, CONEMUCONFIG and possibly others that
the test script didn't show. A quick run of googling pointed me to
https://code.google.com/p/conemu-maximus5/

The test script seems to see these environment variables, but somehow they
are not transmitted along to the child process. I suspect this might a
feature of ConEmu itself.

If I'm right, then my only option for now is to remove these variables
from the test data set before running the test.
explicitely test an empty environment variable
new 'setpgrp' option to enable/disable the call to setpgrp() in the child

discussion at https://github.com/evalEmpire/gitpan/issues/60

the performance gain is probably insignificant most of the time,
but the extra flexibility can't hurt
unindent the Changes file
Changes for version 1.110