Merge branch 'su-runuser-improvements' of https://github.com/calestyo/util-linux

* 'su-runuser-improvements' of https://github.com/calestyo/util-linux:
  man pages: document `--user` option for `runuser`
  man pages: use `user` rather than `username`
  bash-completion: add `--pty` and `--no-pty` options for `su` and `runuser`
  bash-completion: complete `--user` only for `runuser`, not for `su`