acctfuncs.inc.php: Change return type of valid_username function

The function is only determining whether a username is valid,
so it makes more sense to simply return a boolean value.

Signed-off-by: canyonknight <canyonknight@gmail.com>
Replace permission check code with can_edit_account()

Signed-off-by: canyonknight <canyonknight@gmail.com>
Clear a user's active sessions following account suspension

A suspended user can stay in active sessions. Introduce new function
delete_user_sessions to remove all open sessions for a specific user.
Allows suspensions to take effect immediately.

Signed-off-by: canyonknight <canyonknight@gmail.com>
acctfuncs.inc.php: Change wording of account editing message

An error message is printed when the number of affected rows is
0 for an edited account. A count of 0 doesn't imply an error,
only that no changes were made in the database.

Signed-off-by: canyonknight <canyonknight@gmail.com>