Fix load errors related to the new ari_model_validators.

Fix load errors related to the new ari_model_validators.

The Asterisk strategy of loading modules with RTLD_LAZY to extract metadata
from the module works well enough, until you try to take the address of a
function.

If a module takes the address of a function, that function needs to be
resolved at load time. That kinda defeats RTLD_LAZY.

This patch adds some ari_validator_{id}_fn() wrapper functions for safely
getting the function pointer from a different module.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@393576 65c4cc65-6c06-0410-ace0-fbb531ad65f3