add options to load and read my.cnf by the C client

These options can be used to read a config file like /etc/my.cnf or ~/.my.cnf. By default MySQL's C client library doesn't use any config files unlike the client programs (mysql, mysqladmin, ...) that do, but outside of the C client library. Thus you need to explicitly request reading a config file...

This makes the commit d3e3e1ea24338cc8a3d662d41f0669b0558fe082 #60 unnecessary as local-infile can be set in the my.cnf file