xundump: Don't require ID field to come first

xundump: Don't require ID field to come first

The code needs it first so it can store field values into the object
right away.  Save the values instead, and store them when the row is
complete.

The improvement is hardly worth the trouble by itself, but it'll
simplify supporting keys consisting of multiple fields, like sector
xloc, yloc or realm cnum, realm, so we can omit rows in those tables,
too.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>