ifconfig: add report of the string from SIOCGIFDOWNREASON.

Sample output:

ifconfig mce0

mce0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=3ed07bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,TSO6,LRO,VLAN_HWFILTER,VLAN_HWTSO,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6,TXRTLMT,HWRXTSTMP>

ether e4:1d:2d:e7:10:0a
media: Ethernet autoselect <full-duplex,rxpause,txpause>
status: no carrier (Negotiation failure)
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

Reviewed by: hselasky, rrs
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21527