LSM trees don't support named checkpoints: disallow attempts, add
documentation, and change test/format to not attempt it.   Ref #546.
Don't use WT_ERR after the "err" label.
Avoid named checkpoints in test/fops now that they are prohibited with LSM.
Merge pull request #550 from wiredtiger/named-lsm-checkpoints

Disallow attempts to create named checkpoints on LSM trees.