dhcp4: don't use offered address in initial request

Do not send initial request using the offered IP address as source
address or server address in destination, but broadcast on layer 2
and 3 (RFC 2131, Section 3.1.3).
dhcp4: set fsm state before building the message

The state is required to distinguish e.g. between different request
types while building the message, not only to handle responses.
Merge pull request #12 from mtomaschewski/dhcp-request-1

dhcp4 request fixes 1