Set environment variable when mpibind is on

[eleon]

Feature request. Programs may benefit from knowing whether mpibind has been applied. For example, before doing affinity, they could check if mpibind has been applied and avoid binding their workers. When mpibind is not on, they could apply affinity.

When mpibind is called, it could set an environment variable such as MPIBIND_IS_ON=1 Programs could then check whether this variable is set in the environment.

Tagging @benson31 for awareness.