Try to make the use of the eoftoken valid. Given that its value

Try to make the use of the eoftoken valid.  Given that its value
is 0 which was also used as a sentinel in ritem, (i) make sure >= 0
is used instead of > 0 where appropriate, (ii), depend upon nritems
instead of the 0-sentinel.
* src/gram.h, src/gram.c (nritems): New.
Expected to be duplication of nitems, but for the time being...
* src/reader.c (packgram): Assert nritems and nitems are equal.
* src/LR0.c (allocate_itemsets, new_itemsets): Adjust.
* src/closure.c (print_closure, print_fderives): Likewise.
* src/gram.c (ritem_print): Likewise.
* src/print.c (print_core, print_grammar): Likewise.
* src/print_graph.c: Likewise.