Fix DHCP client crash with worker threads

Fix DHCP client crash with worker threads

Crash occurring With a worker thread configured and dhcp
client active on an interface. When a DHCP reply packet
is received, call to ethernet_get_main() from
dhcp_proxy_to_client_input() was causing a crash.
Replaced with a call to vnet_get_ethernet_main().

Once that was resolved, calling dhcp_client_acquire_address()
from a worker thread also caused a crash. Changed so the main
thread will do the address/route configuration.

Change-Id: Ib23984787102dea8cf6cfcde86188a751f15c1e1
Signed-off-by: Matthew Smith <mgsmith@netgate.com>