Historically /etc/netstart (and the equivalent code in the install

Historically /etc/netstart (and the equivalent code in the install
script) did 'ifconfig <if> down' before starting dhclient(8). This was
a way of ensuring old running copies of dhclient were killed before a
new one started. Current dhclient does not need this assist, so change
"ifconfig <if> down" to "ifconfig <if> up" pending further script
optimizations.

Similar to a 2014 attempt by halex@. Prompted by a misc@ report
from Kristjan Komlosi reporting hanging diskless setups.

ok halex@ kn@