move getenv from rcl to here for more reuse

[wjwwood]

Connects to ros2/rmw_fastrtps#79

[dirk-thomas]

I don't think rmw is a good location for those since it has nothing to do with the ROS middleware interface. Maybe a new package for those functions would be a good idea (so that it can be used from rmw implementations as well as rcl*).

[Karsten1987]

btw, should we close this in favor of ros2/rcutils#2 ?

[wjwwood]

@Karsten1987 I'd say that we should use this implementation (documentation, signature, and implementation, if they're not already identical to what's in ros2/rcutils#2) before this is closed. I spent a good deal of time tweaking this implementation to work the same on Windows and Unix without any inconsistencies and avoiding memory allocation if possible.

[wjwwood]

Or at least as nearly as possible, given that this implementation uses RMW_* defines.

[mikaelarguedas]

the implementation in ros2/rcutils#2 will be identical on behaviour and argument type. The return codes will be different though

[wjwwood]

Closing in favor of ros2/rcutils#2