Don't permit convert to spend more money than available

Don't permit convert to spend more money than available

Complicated by the fact that conv() ran the conversion code twice,
first for adding up the cost for chkmoney(), then for actually
converting.  chkmoney() asks the player to confirm when he's about to
spend more than half his cash.  Get rid of that, not worth the
complexity.  This merges do_conv() back into conv().