IPServiceType set to CS6 (network control) causes problems on some old
network setups that continue to interpret the field as IP TOS.
Make DHCP work on such networks by allowing this field to be set to
CS4 (Realtime) instead, as this maps to IPTOS_LOWDELAY.
Signed-off-by: Siddharth Chandrasekaran <csiddharth@vmware.com>