Fix the sqlite3_table_column_metadata() routine so that it gives the correct

Fix the sqlite3_table_column_metadata() routine so that it gives the correct
answer for the "rowid" column in a WITHOUT ROWID table.  Enhance it so that
it can be used to check for the existence of a table by setting the column
name parameter to NULL.  The routine is now included in the build by
default, even without the SQLITE_ENABLE_COLUMN_METADATA compile-time option.

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