Support `printenv`

[certik]

So that I can see which environment variables are defined. This is useful for figuring out what variables to put into my .shellrc to make MSVC work.

printenv is already in coreutils, so we just need to expose it.

[certik]

As a workaround this works:

$ cmd /c set

This prints all environment variables that cmd can see and it seems identical to coreutils printenv.

[wolfv]

Done in #232