Section `3.11 Environment variables` is mainly my fault. As discussed
earlier on this SRFI's mailing list, setting environment variables in
Scheme can have complicated consequences. Hence, I would prefer to
redact the procedures I added earlier (set-environment-variable! and
delete-environment-variable!).

get-environment-variables and get-environment-variable are definitely
useful but they're copied from SRFI 98. Not sure if there's a particular
benefit to having them in SRFI 170 as well. I'm fine with removing them
as well.