Fix regression from r54429 "(bug 17864 + bug 19519) - Do input normalization on the...

Fix regression from r54429 "(bug 17864 + bug 19519) - Do input normalization on the username before doing existence/permission checks"

This did some overactive canonicalization of the input username before username and database were split, breaking access to remote users where the database name contained an underscore -- it was changed to space.
This would also have broken numeric access, I think, but I didn't bother to test that. :)
Fix moves the username canonicalization down into fetchUser() after we split the name and database, so we now can access remote folks via non-canonical forms and we don't break the dbnames.