In order to support CARP interfaces kernel was taught to handle more

In order to support CARP interfaces kernel was taught to handle more
than one interface in one subnet. However, some userland apps rely on
the believe that this configuration is impossible.

Add a sysctl switch net.inet.ip.same_prefix_carp_only. If the switch
is on, then kernel will refuse to add an additional interface to
already connected subnet unless the interface is CARP. Default
value is off.

PR:			bin/82306
In collaboration with:	mlaier