Drush sql regressions

This PR contains four commits to improve regressions on behavior that worked in Drush 6 that is currently broken in Drush 7.  The most important of these is a fix to bootstrapping during sql class selection.  In Drush 6, bootstrapping would work even if the database did not exist, but Drush 7 would fail, e.g. on sql-create of a database that did not already exist.  This PR fixes that, makes --simulate in sql-sync a bit better, and other minor changes.

Travelling; this PR will get Jenkins to run the tests for me, so I don't need to do it over my crummy connection. :)