Enhance sqlite3_shutdown() so that it automatically invokes

Enhance sqlite3_shutdown() so that it automatically invokes
sqlite3_reset_auto_extension().  This is a harmless no-op if applications
are already calling sqlite3_reset_auto_extension() prior to sqlite3_shutdown().
And it prevents possible memory corruption if they do not.  So it works
either way.  Most of the changes are to the test cases. (CVS 6475)

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