env: reject bogus -u arguments

* src/env.c (main): Use unsetenv rather than putenv to remove
items from environ, and check for failure.
* bootstrap.conf (gnulib_modules): Add unsetenv.
* tests/misc/env: Test this.
* NEWS: Document it.

bootstrap.conf

@@ -217,6 +217,7 @@ gnulib_modules="
   unistd-safer
   unlink-busy
   unlocked-io
+  unsetenv
   update-copyright
   uptime
   useless-if-before-free