CSV engine mode for standard-compliant quote and comma parsing

MySQL CSV SE is non-standard with respect to embedded " and , character parsing: http://bugs.mysql.com/bug.php?id=71091 / bug 1316042. Fixing this bug unconditionally breaks MySQL CSV format compatibility for any pre-existing user tables and for data exchange with other MySQL instances, but improves compatibility with other CSV producing/consuming tools.

To keep both MySQL and other tool compatibility, introduce a new dynamic, global/session server variable csv_mode of SET type, allowing an empty value (the default), and IETF_QUOTES.

If IETF_QUOTES is set, then embedded commas are accepted in quoted fields as-is, and a quote character is quoted by doubling it. In legacy mode embedded commas terminate the field, and quotes are quoted with a backslash.

A SET variable is chosen so that new values may be added in the future to accommodate other selective CSV format changes as needed (such as embedded newline character parsing).