Change regular-expression primitives to treat failure-stack overflow

Change regular-expression primitives to treat failure-stack overflow
as equivalent to "no match".  Also, increase the maximum size of the
failure stack from 1000 to 20000.  This is important because it is far
too easy to overflow the stack, the stack-overflow errors cause Edwin
to bomb out.  This is a problem because stack-overflow errors can be
caused by user data -- the proximate example being a News message that
bombed out the News reader because it had a header line was too
complicated to match.  Better to fail in these situations than to
error.