Sql.mysql: Clean up character set handling.

Adds helper functions fix_{query,result}_charset() that contain
code that used to be in the QUERY_BODY() macro. This makes the
code quite a bit easier to read.

Adds charset wrappers to list_{dbs,tables,fields}() that previously
changed behavior depending on what the send_charset was.

Remove some now obsoleted character set handling from Mysql.SqlTable.

Potential fix for intermittent failures for Mysql.SqlTable to handle
tables with wide characters in the table name.