[invoker] Don't add POSIXLY_CORRECT to the environment

As a GNU extension, passing '+' to getopt_long can be used as an
alternative to the POSIXLY_CORRECT environment variable to get the
argument behavior that invoker relies on. Variables set in the invoker
environment will pass onto child processes, and adding this to the
environment would break some applications.
Merge pull request #26 from special/getopt-posixly-correct

[invoker] Don't add POSIXLY_CORRECT to the environment