zsh: update varlinkctl completions

zsh: update varlinkctl completions

correct redundant or mismatched tags and fill the argument field of
curcontext because _regex_words does not do that for us.

The _complete_help text now looks much more reasonable most of the time:

$ varlinkctl call /run/systemd/resolve/io.systemd.Resolve ^Xh
tags in context :completion::complete:varlinkctl::
    argument-rest  (_arguments _varlinkctl)
tags in context :completion::complete:varlinkctl-call:method:
    varlink-methods  (_varlinkctl_cmd _varlinkctl_command _arguments _varlinkctl)

Fixes: af63b4b769bf ("zsh: add varlinkctl completions")