MINOR: hlua_fcn: add Patref:event_sub()

MINOR: hlua_fcn: add Patref:event_sub()

Just like we did for server events, in this patch we expose the PAT_REF
event family (see "MINOR: event_hdl: add PAT_REF events") in Lua.

Unlike server events, Patref events don't provide additional event data,
and the registration can only take place from a Patref object (ie: not
globally).

Thanks to this commit it now becomes possible to trigger actions when
updates are performed on a map (or acl list) being monitor, without
the need to loop or use inefficient workarounds.