compat: support dummy interface

Parsing of ifcfg files belonging to dummy interfaces.
ifconfig: provide dummy system functions

System functions for creating, changing and deleting dummy interfaces.
ifconfig: return netlink error code if link create fails

Since the errors are returned in their negative forms, current checks
for 0, less than 0, etc, work fine.

This is currently useful for Dummy interface creation since loading the
dummy kernel module automatically creates a dummy0 interface. If the user
tries to create another via ifcfg/xml, checking for -NLE_EXISTS allows us
to better handle this case from ni_system_dummy_create().
dbus: dummy interface dbus service

Support for creating dummy network interfaces.
dbus: enable dummy dbus service
Merge pull request #192 from kmroz/dummy-rc2