DHCP: Fix crash

Rarely (about once every 20 runs) we had OSV crash during boot, in the
DHCP code. It turns out that the code first sends out the DCHP requests,
and then creates a thread to handle the replies. When a reply arrives,
the code wake()s the thread, but on rare occasions the thread hasn't yet
been set up (still a null pointer) so we have a crash.

Fix this by reversing the order - first create the reply handling thread,
and only then send the request.