The separation between parsing and semantic actions is awkward and
leaks memory; rework it:
(deffld): New; semantic action for field headers.
(xufldname): Use it instead of storing a value for xucolumnheader().
(xucolumnheader): Remove, use xuflds() directly instead.
(setnum, setstr, setsym): New; semantic actions for field values.
(xufld): Use it instead of storing a value for xuloadrow().
(xuloadrow): Remove. Was incomplete.
(xufldname, xufld, xuflds): Remove parameter values. Callers
changed.
(cur_type, cur_obj, nxt_sel, nxt_idx, nxtfld, ca4fld, getsymtab): New
helpers.
(xundump): Initialize them.
(enum_value, value, freeflds, xunsymbol): Remove.
(mtsymset, add2symset): New.
(xufld): Use them to build a symbol set along the parse.
(xufldname, xufld, xuheader, xutrailer, xundump): Increment lineno
when and only when a linefeed is consumed, to make sure it is correct.
Move some semantic checks so that errors get reported for the correct
line.
(xundump): Use compare filenames as pointers, not strings, when
checking whether to reset lineno.
(xundump): Keep empfile[type] consistent during parse, to make
ef_ptr() usable.
(xundump): Don't call ep->init() on sentinels.
(xuinitrow): Unused, remove.