Fold in some startup randomness to make the hash vary across
different runs.  This helps prevent attackers from choosing
keys that all map to the same bucket.

Also, reorganize the hash a bit.  Move the *m1 multiply to after
the xor of the current hash and the message.  For hash quality
it doesn't really matter, but for DDOS resistance it helps a lot
(any processing done to the message before it is merged with the
random seed is useless, as it is easily inverted by an attacker).

Update #9365

Change-Id: Ib19968168e1bbc541d1d28be2701bb83e53f1e24
Reviewed-on: https://go-review.googlesource.com/2344Reviewed-by: Ian Lance Taylor <iant@golang.org>