bug#63411: 29.0.90; pcomplete/systemctl cannot complete units for old versions of systemd

[Liu Hui]

Hi,

It is very convenient that Emacs 29 supports eshell completion for
systemctl, but I find units cannot be completed on Ubuntu 20.04 LTS:

1. emacs -Q
2. M-x eshell
3. type "systemctl stop ", and press TAB

There is no completion for units.

The reason is that 'pcmpl-linux--systemd-units' uses a new option
'--legend=no' that is introduced since Feb 18, 2021[1], and the option
cannot be recognized by systemd v245 on my system. Since the old
option '--no-legend' is still kept for compatibility[2], I suggest we
can just replace '--legend=no' with '--no-legend' to make systemctl
completion works for old systems.

[1] https://github.com/systemd/systemd/pull/18596
[2] https://github.com/systemd/systemd/blob/main/src/systemctl/systemctl.c#L485

Best,
Liu Hui