Added. Harden default database table prefix with a random string

git-svn-id: https://plugins.svn.wordpress.org/sucuri-scanner/trunk@948278 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added. Warning message for the database table prefix hardening

git-svn-id: https://plugins.svn.wordpress.org/sucuri-scanner/trunk@948280 b8457f37-d9ea-0310-8a92-e5e31aec5664