Problem in rt2-to-rt3 import script

Having put off upgrading from rt2 to rt3 long enough, I took the plunge
this morning.

However after importing the data into RT3 I found a serious problem. None
of the Attachment owners were correct.

Traced the problem to the dumpfile-to-rt-3.0 script which wasn’t updating
the Attachment Creator id with the new one from user_map.

Here’s the patch.