1.  NCCRUNE was too large by sizeof(Reclass.end)/sizeof(Rune).
use enum of NSPANS in <regexp.h> to size Reclass.spans
and NCCLASS to size Reprog.class.

could use nelem(yyclassp->spans) and nelem(classp->class)
instead, but that seems a tad confusing.

2.  a too-large character class didn't compile correctly and did not
yield an error.  error out if character class is too large.

3.  use NSPANS=128 and not NSPANS=64/2.