Add a sysctl `net.link.tap.deladdrs_on_close' to configure whether tap
should delete configured addresses and routes when the interface is
closed.  Default is enabled (preserve current behaviour).

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274966 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f