callout on property change

[Robert Love]

Hello, guys.

I added callout-on-property-change, because we need it to do lots of
things, such as a replacement for ifplugd (take action on link change).
See attached patch.

Couple of design decisions:

        - I only invoke the callout when the property is _modified_, not
        when it is first added or removed.  I don't see the need for
        added or removed property callouts, do you?  This gets rid of a
        few spurious callouts during HAL init, too.  What people want, I
        think, is to catch the modification and do something based on
        the new vs. old value.
        
        - Despite the above, I pass "modified" as the first parameter to
        the callout.  This is in case we ever want to change the above
        assumption - also, it remains compatible with the parameters of
        the other callouts.
        
        - I added an environment variables PROPERTY (the key that
        changed) and VALUE (the key's current value).  It is too much to
        expect the callouts to translate the PROPERTY into the right
        format (dots to underscores, all uppercase, etc.) and then
        invoke the result as a variable.  So I pass them.
        
        - However, VALUE is currently hardcoded to foo until I write
        hal_property_to_string().

I know there were some concerns about properties changing too often for
a callout to remain sane.  In my testing, however, there were
surprisingly few property-modified callouts occurring - perhaps since I
only report when a property changes (as opposed to add/remove)?  During
my testing, the only property change callouts I saw were me pulling out
my ethernet cable to test.  So who knows.

Comments, thoughts?

	Robert Love

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