Adding TRUNC builtin.

Includes additions to builtin UDF registration to support prepare/close.

Change-Id: I22668fa7ee033b3fa37050b7bccee935571ac453
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2141
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins