dumpon: warn if the configured netdump link is down

Previously we expected the DIOCSKERNELDUMP ioctl to return ENXIO if the
interface was down, but it does not actually do this. Grab the link
status using getifaddrs(3) instead, and downgrade this case from an
error to a warning; the user might bring the link back up at a later
time.

Reviewed by: cem
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35196