Add an ifconfig carp option that enables users to set

Add an ifconfig carp option that enables users to set
the state of the carp cluster.

This is a direct commit to stable/9 because -HEAD's
code is very different. I discussed this with Gleb
and the reason for this is that since we do not
touch the kernel itself and are not adding very
weird or confusing things, we can commit this to the
stable branch directly.

The options 'master' and 'backup' are now available,
which enables the administrator to force a node into
the backup or master state on the cluster. Ofcourse
preempt has to be disabled otherwise the master node
will become master again.

One can do that with:

sysctl net.inet.carp.preempt=0

After that one can schedule maintenance on the node
normally running as the master and such.

PR:		100956
Discussed with:	glebius
MFC after:	1 weeks

git-svn-id: svn://svn.freebsd.org/base/stable/9@232486 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f