dhcp4: do not release and remove lease without request

The request/config defines if the lease has to be released
or not on a drop request. Do not remove lease file when not
requested.
dhcp4: do not use compat but wicked variable names
Merge pull request #400 from olafhering/olh-misc

Fix compile error with ni_sockaddr_prefix_parse calls
Merge pull request #401 from mtomaschewski/dhcp-no-release

dhcp4: do not release and remove lease without request (bnc#866994)