We never touch ulen.  If DB_DBT_USERMEM and ulen is too small, we set size, (do not write anything)
and return DB_BUFFER_SMALL.

This includes test_db_dbt_mem_behavior.c
Closes #146

git-svn-id: file:///svn/tokudb@1034 c7de825b-a66e-492c-adef-691d508d4ae1