Add getenvs, update getenv, change extract_path use

Stacks of modules were using `extract_path` where it wasn't really semantically correct
because this was the only way to expand environment variables. This commit fixes that
up a bit.

Also, I changed the existing `getenv` function in `stdapi` to `getenvs`, and had it
support the splat operator. I added a `getenv` function which is used just for a
single variable and uses `getenvs` behind the scenes.

The meterpreter console `getenv` command now uses `getenvs`