More helpful error messages in case of conflicting triggers

Closes issue #5.
Imply --no-order for non-clustered tables

--no-order can still be specified to VACUUM FULL a clustered tables
(not so useful I guess...)

Fixes issue #6.
Options sorted in a slightly more rational order

--no-order now is almost useless, but list it next to --order-by.
--jobs only specifies how to do something, not what to do. On the
same basis probably --no-analyze should be pushed further up.