In order to perform a smooth transition from NIC drivers that act only as NIC
session clients to NIC drivers that act only as Uplink session clients, this
commit introduces an intermediate state in which all NIC drivers support both
modes. That said, a NIC drivers mode is now statically determined through a new
optional 'mode' attribute in the drivers <config> tag that can be set to either
'nic_server' (default value) or 'uplink_client'. Reconfiguring this attribute
at a driver doesn't have any effects. Whithout this attribute being set, all
NIC drivers will behave the same as they did before the commit. When set to
'uplink_client', however, instead of providing a Nic service, they request
an Uplink session whenever their network interface becomes "UP" and close the
session whenever their network interface becomes "DOWN".
Ref #3961
