As reported in [1], completions provided by "set global completers
option=my_completion" activate insert mode autocompletion, even when
the autocomplete option does not have the insert mode flag.
This happens because InsertCompleter::on_option_changed() calls
InsertCompleter::setup_ifn(), which shows the completion pager.
Fix this by computing whether the completion pager is enabled;
otherwise we can return early from setup_ifn().
The completion pager is enabled if the autocompletion bit is set,
or if the user has requested explicit completion.
[1]: https://github.com/kak-lsp/kak-lsp/issues/585
