fix various issues with IpAddr interface

* prevent copy-initialisation with integer values
  (maybe not what we want to do)
* remove by-pointer ctor API
  (we don't need nullptr case)
* fix constness misuse on sockaddr conversion methods
* fix constness of bool conversion
  (for cases where the IpAddr instance is not const)
* fix IpAddr rich-comparators (add <=, >=, <, >)
  (fix usage of IpAddr as key in a map, where
  the comparator is std::less, operator== was not used)

Change-Id: Iba9cf641d68bda37bd32d85dbd9dfffdc99078ed
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>