Rename module 'putenv' to 'putenv-gnu'. * modules/putenv-gnu: Renamed from modules/putenv. (Description): Mention the removal of environment variables. * modules/putenv-gnu-tests: Renamed from modules/putenv-tests. * modules/putenv: New file, an indirection to the new module. * doc/posix-functions/putenv.texi: Mention the new module name. * NEWS: Mention the change.