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