BEGIN WORK and transaction isolation level

This applied patch adds transaction isolation level to BEGIN WORK just
like START TRANSACTION. The bottom of the patch improves table
completion for transaction commands.

This was discussed and approved a while ago. (Peter objected, though.)

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073