Tighten control of things that look like they might be symfuns

"Rational(2, 3)" could not  be a symfun, don't allow the vars to start
with digits.
sym: stop using "rats", it can return "*" sometimes.

Use [n, d] = rat(x) instead, more robust.  Fixes #121.
message for NEWS file
Merge pull request #125 from cbm755/fix_121

just one rat