I've had a look at many of the links provided, and had a look a some of the dictionary entries, where I found the following:
4. (Law) Legally qualified; possessed of legal competence;
as, able to inherit or devise property.
for 'able', as being akin to habile through having the same roots:
L. habilis that may be easily held or managed, apt, skillful,
fr. habere to have, hold. Cf. Habile and see Habit.]http://dictionary.die.net/ablest
in my opinion, the use of 'habile' in the context of baronies and salmon fishing rights represents a further meaning of 'habile' not covered in 'normal' dictionary entries, and being equivalent to 'able' in the sense quoted above: of or having legal qualification.
If my barony is habile for a river, then I have the legal qualification to fish the river.
perhaps this is the reason that there are examples of habile abroad in the world that are not covered by the usual dictionary entries - legal language in BE at least abounds in terms and phrases long obsolete in normal idiomatic English.