Drop received packets unless IFF_RUNNING is set. When hvn(4) attaches

Drop received packets unless IFF_RUNNING is set.  When hvn(4) attaches
it sends commands and waits for replies to come back in.  The receive
pipe seems to contain both command completions and data packets.  When
waiting for command completion during hvn(4) attach, it's possible for
packets to show up as well.  We shouldn't call if_input() if hvn(4) is
not set up, so drop them when we're not running.

ok mikeb@