Add nqp::shell3 that takes  and %env. Remove previous attempt at env.
Merge branch 'master' of github.com:perl6/nqp into shell-rework
Merge branch 'master' of github.com:perl6/nqp into shell-rework
Merge pull request #112 from perl6/shell-rework

Add nqp::shell($cmd, $dir, %env) op and support existing nqp::shell($cmd) until rakudo catches up