Fix that userid cannot be changed by admins.


git-svn-id: http://opensvn.csie.org/pttbbs/trunk@5908 63ad8ddf-47c3-0310-b6dd-a9e9d9715204
Validate input userid; always log userid changes.


git-svn-id: http://opensvn.csie.org/pttbbs/trunk@5909 63ad8ddf-47c3-0310-b6dd-a9e9d9715204