- Use existing functions mtx_lock() and mtx_unlock().

- Use existing functions mtx_lock() and mtx_unlock().
- Change variable name to 'error', as this is what is mostly used for
  functions that return an error.
- Add mutex(9) to the SEE ALSO section.
- Bump the date.

I don't really like the example code. I'd prefer symmetry where possible, eg.

	mtx_lock(&example_lock);
	error = example(NULL, EXAMPLE_ONE);
	mtx_unlock(&example_lock);
	if (error != 0)
		return (error);

But I'll leave it as it is for now.

Reviewed by:	simon