BUG/MINOR: lua: warn when registering action, conv, sf, cli or applet multiple times

BUG/MINOR: lua: warn when registering action, conv, sf, cli or applet multiple times

Lua allows registering multiple sample-fetches, converters, action, cli,
applet/services with the same name. This is absolutely useless since only
the first registration will be used. This patch sends a warning if the case
is encountered.

This pach could be backported until 1.8, with the 3 associated patches:
 - MINOR: actions: Export actions lookup functions
 - MINOR: actions: add a function returning a service pointer from its name
 - MINOR: cli: add a function to look up a CLI service description

(cherry picked from commit f67442efdb509d5d15f530a536b13f29fa7f48b7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 04cbfd7cc0138575caf53e9710a49f1ed0a2ebbd)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit a9bca65df7674d2091ad753bad4e689d76a4a9db)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>