update sys/share/{dgn,lev}_lex.c

update sys/share/{dgn,lev}_lex.c

The HP C compiler for VMS issued a pair of diagnostics for both *_lex.c
files, about an expression of the form (unsigned_var <= 0) maybe not
being what was really intended, and it was right.  Changing that to
'< 1' would have suppressed the diagnostic but left the bug, which was
in code that performed a subtraction and then checked for a negative
result.  It worked in older flex versions when the variables were
signed, but got broken when they were changed to unsigned (no doubt in
respose to gcc complaining about comparing signed and unsigned in some
other spot where one of them was used).

The bug is in flex's original skeleton, so was inherited by our new
custom skeleton.  I've fixed it in the custom skeleton, which means
that sys/share/*_lex.c became out of date even though util/*_comp.l
remain unchanged.