Improve checks for MySQL user's name.

As per http://dev.mysql.com/doc/refman/5.5/en/identifiers.html , MySQL
allows for more than '\w-'. This commit improves the check to ensure
that:
 - if username only contains [0-9a-zA-Z$_], it might be quoted. It is
   not a requirement though
 - if username contains anything else, it MUST be quoted

I kept 2 checks, but the 2nd one can probably be removed (I can't find a
username which match the 2nd one but not the first.)
Merge pull request #588 from maxenced/fix-mysql-user-allowed-char

Improve checks for MySQL user's name.