Allow selection of MySQL storage engine at install time

For reference, see: http://lists.drupal.org/archives/development/2007-06/msg00037.html

This is an issue for placing a database storage engine selection at install time. This would be stored in a site-wide variable which the Schema API would then consult when creating a table.

Problem areas:
a) Where to store this variable before we have a database to put it in? ;) Installation will need to cache this until the database is initialized.
b) We won't know what database engines a host supports until we attempt to connect to it. This means we're offering options which may result in an error when a connection is attempted. Maybe some JS here? Not sure...
c) Which storage engine should be the default? InnoDB offers many benefits, including row-level locking (so your whole users table doesn't freeze up when someone logs in), improved data integrity, and better write performance. However, it's not supported by all hosts (in an informal survey in #drupal, DreamHost, GoDaddy, and Site5 do support it, though LunarPages and MediaTemple don't) and read access is not as performant as MyISAM (for cases when table locks are not occurring).
d) How to handle display of this selection, since it has no bearing on non-MySQL databases?

What might work best is a simple select box under Advanced options:

MySQL Storage Engine:
* default
* InnoDB (recommended)
* MyISAM
(insert thoughtful description of what these changes mean and that they're ignored for non-MySQL users)

"default" is the default, to keep the same behaviour that we have currently.

If InnoDB is selected and it is found not to be an available storage engine, it should be removed from the drop-down when the error is kicked back.