without subsequently doing anything with it, so commit_address()
works with an address with phase == 0. phase == 0 is not allowed,
hence
random84# ifconfig nfe0 atalk 3.14
ifconfig: SIOCAIFADDR: Invalid argument
Solution: copy the new sockaddr_at to a temporary input environment,
and pass that environment to commit_address(). I may as well copy
the new sockaddr_at to the output environment, while I am at it.