MINOR: actions: add a function returning a service pointer from its name

MINOR: actions: add a function returning a service pointer from its name

This function simply calls action_lookup() on the private service_keywords,
to look up a service name. This will be used to detect double registration
of a same service from Lua.

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

(cherry picked from commit 87e539906b13ed1d86684d9d3eab82b550fae02d)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>