check_users: Use utmpx(5) only if available

For systems that don't provide an utmpx(5) interface, restore the code
that was replaced in commit 3e622f3a47bc7d31f22513a79892c3c52febd2d3.
Check for who(1) only if utmpx(5) isn't available

The who(1) command is only called by check_users if the system doesn't
provide an utmpx(5) interface.
Add new Gnulib files to .gitignore