library/io.m:
Use setenv() and _wputenv_s() to set environment variables
on C backends. Unlike putenv() and _wputenv(), these can be
relied upon to copy their inputs, so the inputs can safely
be garbage collected after the call returns.
tests/hard_coded/Mmakefile:
tests/hard_coded/setenv.{m,exp}:
Test case. Only run in grades for which io.set_environment_var/4
succeeds.
