[lmi] XRC RNG validation is incomplete

[Greg Chicares]

Vadim--Commit 2d08538d436 demonstrates a defect in an XRC file that
is obvious when running lmi, but wasn't caught by 'xrc.rnc'; should
it have been? I believe the answer is "no", because the offending
line:

-                        <growablerows>0,1,2,3,4,5,6,7</growablerows>
+                        <growablerows>0,1,2,3,4,5,6</growablerows>

was just as "valid" in the RNG sense without that correction--i.e.,
RNG can test whether "0,1,2,3,4,5,6,7" is _in form_ a comma-separated
list of plausible nonnegative integers, but not whether "7" makes
sense _in context_. But I figured it wouldn't hurt to ask anyway.