zsh: reintroduce pattern argument to uncached verbs

zsh: reintroduce pattern argument to uncached verbs

The systemctl completion previously made use of PREFIX as a pattern
argument to list-unit-files and list-units. This had the problem of
erroneously filtering the results that were stored in the cache, and
erroneously filtering results that might have been requested according
to the users configuration (e.g. _correct completer, certain
matcher-lists or tag-orders, etc.).

Unfortunately, the runtime of list-unit-files increases when no pattern
argument is provided, and systemctl show, used to filter those units,
can become unacceptably slow when provided with too many units to
describe.

Let's re-introduce the pattern argument to list-unit-files and
list-units where necessary in order to alleviate these bottlenecks
without poisining the cache. A 'use-pattern' style is introduced that
may be used to disable this behavior if it is undesired. We can still
expect that certain completions, like `systemctl start <TAB>` will be
slow, like before. To fix this we will need systemd to learn a more
efficient way of filtering the units than parsing systemctl show.