should `addenv` merge PATH and LD_LIBRARY_PATH?

[simeonschaub]

I would have expected addenv(cmd, "PATH" => "foo:bar") to add foo and bar to cmd's path if it has already been set. That might be a breaking change now though?

[DilumAluthge]

Definitely breaking to add to addenv, because the docstring says "Duplicate keys are replaced", which means that PATH is replaced if it already exists.

But we could either:

1. Make a new function.
2. Add a new kwarg to addenv.