[relax-ng] externalRef v external; notAllowed

I've slept on John C's suggestion to replace the "externalRef" keyword by
"external" and I agree with it.

One reason is that I find mixedCase keywords a bit strange.  Making this
change leaves us with only one mixed case keyword, "notAllowed", which, to
my mind, now sticks out like a sore thumb.  But I'm not sure what to do
about it. Does this bother anybody else?  Alternatives that occurred to me
were:

1. none
2. (|)
3. notallowed
4. not allowed
5. 0
6. {}

But I'm not sure if any of these are an improvement.

James