Support vertically split configuration tables (human-readable only):
(ellipsis): New.
(xuheader1): Initialize it. Compute nflds. Allow repetition of index
field in continued tables.
(xundump): Don't set nflds.
(xufldname, defellipsis): Accept ... and call new defellipsis().
(xufldname, chkflds): Update and elaborate check for missing fields,
move it to new chkflds(). Require repetition of index field in
continued tables.
(xundump2): New, factored out of xundump(). Parse continued tables.