MINOR: cli: add a function to look up a CLI service description

This function will be useful to check if the keyword is already registered.
Also add a define for the max number of args.

This will be needed by a next patch to fix a bug and will have to be
backported.

(cherry picked from commit a51a1fd17420a96bb766afbae354e041fc9e1d9b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
3 files changed