[config] Streamline extension runtime dir creation

1. Use /var/run/wicked/extension/EXTENSION_NAME
2. Do the creation from ni_extension_statedir(), rather than adding
extension specific functions to config.
[system_update] Hostname update from lease

Update system hostname from first arriving lease with hostname field set,
and emit hostname updated signal. Subsequent leases not allowed to set
hostname unless:
  i. lease that set hostname has been removed
  ii. arriving lease is an update