Remove the non-portable `function`.

`function` isn't a reserved word in dash.  Remove it.
Don't add `${NENV_ROOT}/shims` twice if it's already been in `PATH`.
Merge pull request #3 from dochang/non-portable-function

Remove the non-portable `function`.
Merge pull request #4 from dochang/duplicate-shims

Don't add `${NENV_ROOT}/shims` twice if it's already been in `PATH`.