Add optional host/IP parameter to User functions

As a RedDwarf user, I need to be able to optionally specify a hostname for database users in all API calls, so that my instance is secure and not available to everyone with network access that has the right username and password.

In every case where the user does not supply a hostname, the hostname is defaulted to '%', MySQL's shorthand for 'any host'."

Additionally, a configuration flag should be supplied to optionally reject API requests bearing non-IP hosts, in the case that hostname checking is disabled on the instance. Specifying a hostname (instead of an IP) for a user while hostname checking is disabled will cause MySQL to shut out otherwise legitimate connections, as there won't be any matches in mysql.users with the source IP.