(derives_compute): Do not subtract NTOKENS from

the pointer DSET returned by malloc; this isn't portable.
Instead, always use DSET[i - NTOKENS] rather than DSET[i].
Similarly for DERIVES.

(derives_compute): Do not bother invoking
int_of_rule_number, since rule numbers are integers.

All uses of XCALLOC, XMALLOC, and XREALLOC changed to use new macros
CALLOC, MALLOC, REALLOC.  All uses of XFREE changed to free.