Correction for return codes, --systemd flag and fixes

This one is supposed to address issue #112.

It consists of:
* Add new enums for LSB return and status codes
* Make use of the above enums within autoip4, dhcp4, dhcp6, server, nanny, client
* Introduce --systemd common options for autoip4, dhcp4, dhcp6, server, nanny, client - this options is suppose to redirect log desination to syslog as well as surpress returning error codes systemd does not like.
* A few small fixes