supply "detect" option for passive_updates

[Mike Bayer]

with #1362, the passive_updates flag no longer has a sqlite/myisam/everything else compatible setting. an auto-detect based on sqlite/myisam table should be added, document that "myisam" is detected via mysql_engine.

[Mike Bayer]

this is really not needed at all, mutable primary keys are already a rare edge case, and then the cases where "passive_updates=False" actually creates a problem on an FK database is very rare, essentially there's one test in test_naturalpks that illustrates this. there's not an easy way to detect this condition in any case as on MySQL is per-table. not really worth it.