systemd: add knot-resolver alias

The `knot-resolver` name is used for paths and user name. Creating a
systemd alias with the same name is user-friendly to end users who won't
have to remember another name (`kresd`).

Note: Systemd Alias is only created after service is enabled. Packagers
are thus advised to create symlinks for unit files during package installation
so users can use `knot-resolver` name right from the start.