ifupdown: add new 'down [yes|no]' link attribute to keep link down

ifupdown: add new 'down [yes|no]' link attribute to keep link down

Ticket: CM-13434
Reviewed by: julien, nikhil, daniel
Testing Done: ifreload and multiple down [yes|no] sequences under
physical and logical interfaces (ifupdown2-tests test case is pending)

This also moves the fix done for CM-4125 (inet manual handling for
logical devices) into a single place under ifupdownmain.

attribute 'down [yes|no]' will not work in all cases when 'inet manual'
is used. This is only to preserve the semantics of 'inet manual'.
Best use of 'down [yes|no]' is to use it without 'inet manual'..
they are conflicting features anyways.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>