systemctl: Use /usr/bin/editor if available

systemctl: Use /usr/bin/editor if available

If the EDITOR environment variable is not set, the Debian policy
recommends to use the /usr/bin/editor program as default editor.
This file is managed via the dpkg alternatives mechanism and typically
used in Debian/Ubuntu and derivatives to configure the default editor.

See section 11.4 of the Debian policy [1].

Therefor prefer /usr/bin/editor over specific editors if available.

[1] https://www.debian.org/doc/debian-policy/ch-customized-programs.html