Clean-up: Adding a class Term_string to share some LOAD DATA code

The new class is used for:
- FIELDS TERMINATED BY 'string'
- LINES STARTING BY 'string'
- LINES TERMINATED BY 'string'

The implementation of "FIELDS ENCLOSED BY 'char'" and "FIELDS ESCAPED BY 'char'"
should eventually also switch to this class to support multi-byte characters.
Currently multi-byte ENCLOSED and ESCAPED characters are rejected at parse time.