tap: allow change of carrier state on host

tap: allow change of carrier state on host

Type: feature

Add a function to adjust the link state of the host side of a tap
interface. If an application (e.g. route protocol daemons) running on
the host uses netlink to monitor interface state, a plugin
could use this function to communicate a loss of connectivity
to the application by making the interface appear to go down.

Requires a somewhat recent kernel. E.g. it does not have any effect
on CentOS 7 but it works on CentOS 8.

Change-Id: I677ee7889d2eb142e2395bea98f0b4d7e7e7f810
Signed-off-by: Matthew Smith <mgsmith@netgate.com>