mysql_user module: adding support for various REQUIRE options.

This is meant to be a replacement for the incomplete and now closed PR #7744 .

This PR introduces 5 new parameters:
- require_ssl
- require_x509
- require_subject
- require_issuer
- require_cipher

More information about their purpose can be found [here](http://dev.mysql.com/doc/refman/5.1/en/grant.html).

Personally, I've only ever used 'REQUIRE SSL' in MySQL queries, but some might want more fine-grained control, hence the other 4 parameters.