zsh-completion: _loginctl/_systemd/_systemd-inhibit improvements

zsh-completion: _loginctl/_systemd/_systemd-inhibit improvements

_loginctl: respects the verbose style. which allows a user to get
the pre d5df0d950f8bc behavior of not showing a description for sessions
and users, by default they aren't shown.

zstyle ':completion:*' verbose true
or
zstyle ':completion:*:loginctl*:*' verbose true # or similar
Will show the descriptions.

zstyle ':completion:*' verbose true
and
zstyle ':completion:*:loginctl*:*' verbose false # or similar
Won't show descriptions for loginctl only

_systemd: complete pids for systemd-notify's --pid option.
display a message of the expected argument for other options.

_systemd-inhibit: complete block & delay for --mode
display a message of the expected argument for --who/--why