'enabled' attribute - revised version

Hi,

For the record, this looks good to me, thanks.

I've a mild preference for skip_alternative taking the old contraint
position and returning the new position, rather than taking a pointer
to the position and adjusting it in-situ. But maybe that's just me.

I did it that way due to the const char* vs. char* conflicts between
reload and recog. Without modifying the pointer in-place the code
would remove the const qualifier by either casting or parameter
passing.

The markup of:

`enabled'

doesn't look right. Texinfo quotes are `` and '' (like TeX),
but existing attribute docs seem to use @code{} instead.

Fixed.

Mark, is that ok to apply to mainline with the documentation fix above?

Bye,

-Andreas-