Removed global exception handler.

It was moved into ruby bindings, so not needed anymore.
Implemented basic setup of DHCLIENT_SET_DEFAULT_ROUTE
Refactoring: LanItemsClass#Commit - dhclient setup
Added setters for sysconfig values.
Refactoring: simplified workflow in LanItemsClass#Commit
Set DHCLIENT_SET_DEFAULT_ROUTE in network autoconfiguration. bnc#868187
Refactoring: LanItemsClass#AddNew
DHCLIENT_SET_DEFAULT_ROUTE backward compatible setup for "normal" mode
Updated changelog
Dropped handling of obsoleted or unsupported sysconfig values.

TUNNEL_SET_PERSISTENT, DHCLIENT_SET_DOWN_LINK.

See https://github.com/yast/yast-network/pull/189 for details.