Allow to multiple arguments in system-updater run
Rework updates in preparation for netconfig

1. Remove concept of lease weights, allowing netconfig policies to decide.
2. Handle multiple lease updates.
3. Introduce ni_system_updater_remove() for removal of lease information
   (mainly used by netconfig). Note that full functionality of this
   feature will rely on extension/resolver changes in upcoming patch.
[extensions/system_update] Enable netconfig support for resolver extension

If available, extensions/resolver will use /sbin/netconfig for dns updates.
Otherwise, we fall back to mananging /etc/resolv.conf in the initial
fashion.

Also, incorporating the following fixes/enhancements provided by mt:
- Fixed resolv.conf data parsing in netconfig mode.
- Under netconfig, the netconfig update policies are used.
  Otherwise the extension implements own external policy to
  prefer settings, e.g. static over dhcp or ipv4 over ipv6.