useradd: fix subuid allocation

useradd: fix subuid allocation

closes #1683

The decision on whether to allocate a subuid is dependent
on user_id != 0, but user_id is not filled in until after
the call to this.  I spent a few minutes trying to track
down where this ordering changed, and haven't found it yet.
We can add a fixes tag if we track it down.

Move the decision on whether to calculate it to after we
calculate user_id.  Then, because that decision is used to
decide whether to open the subuid_db, and pull the subuid
db opening out of open_files and run it later.

Signed-off-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Alejandro Colomar <alx@kernel.org>