Report unique usernames for isc_info_user_names

[mrotteveel]

For administrators, isc_info_user_names reports the usernames of each connection. If multiple connections have the same username, then that name is repeated. Instead, it should report the unique usernames; this will reduce the amount of information transferred, reducing the chance of truncation.

Making this change should not break the API, as it makes no promises beyond reporting the usernames, though it might be good to ensure the username of the current connection remains the first as it is currently.

Specifically, to quote from the InterBase 6.0 API Guide:

Names of all the users currently attached to the database; for each such user, the result buffer contains an isc_info_user_names byte followed by a 1-byte length specifying the number of bytes in the user name, followed by the user name