Call cpu_nap() to avoid high CPU load in the dhcp client

This is especially important in virtual machines as waiting
for responses from the server, if it is temporarily not
available, causes 100% CPU load on one of the host's cores.
This happens a lot if all machines including the dhcp server
are running in VMs.

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>