Deprecate CI_Input::is_cli_request() and add common function is_cli() to replace it

Calls to this function are often needed before the Input library is available
diff --git a/user_guide_src/source/general/cli.rst b/user_guide_src/source/general/cli.rst
index 4145d5c..4f3b07d 100644
--- a/user_guide_src/source/general/cli.rst
+++ b/user_guide_src/source/general/cli.rst
@@ -23,7 +23,7 @@
 
 -  Run your cron-jobs without needing to use *wget* or *curl*
 -  Make your cron-jobs inaccessible from being loaded in the URL by
-   checking for ``$this->input->is_cli_request()``
+   checking the return value of :func:`is_cli()`.
 -  Make interactive "tasks" that can do things like set permissions,
    prune cache folders, run backups, etc.
 -  Integrate with other applications in other languages. For example, a