Don't permit demobilize to spend more money than available

Don't permit demobilize to spend more money than available

Complicated by the fact that demo() ran the demobilization code twice,
first for adding up the cost for chkmoney(), then for actually
demobilizing.  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_demo() back into demo().

It also removes the command's virtually undocumented fourth argument.
Update player_coms[] accordingly.  While there, make it require money;
it won't do anything useful without money anyway.