Don't allow changing sid from rehash.

Not sure how this was supposed to originally work, if the sid is changed
the uid generator is not re-initied, and even if it was it would allow
id collisions if it ever uplinked to another ircd with the old id it
had.

I see no reason for this.
Merge pull request #8 from Adam-/unreal34+uid

Don't allow changing sid from rehash.