It's becoming more common that OSes include "ip" but not "ifconfig", so
it's best to avoid using the latter.  This commit removes most references
to "ifconfig" and replaces them by "ip".  It also adds a build-time check
to make it harder to introduce new uses of "ifconfig".

There are important differences between "ifconfig" and "ip":

- An "ifconfig" command that sets an IP address also brings the interface
  up, but a similar "ip addr add" command does not, so it is often necessary
  (or at least precautionary) to add an "ip link set <dev> up" command.

- "ifconfig" can infer a netmask from an IP adddress, but "ip" always
  assumes /32 if none is given.

- "ifconfig" with address 0.0.0.0 removes any configured IP address, but
  "ip addr add" does not, so "ifconfig <dev> 0.0.0.0" must be replaced by
  "ip addr del" or "ip addr flush".

Signed-off-by: Ben Pfaff <blp@ovn.org>