nic bridge: connect to backend NIC on startup

After announcing the NIC service, the bridge connects to the driver to
ensure to see any incoming traffic in case the client itself only reacts
on connects from LAN (e.g., the netperf server).

Also, some styling issues were fixed.
netperf: enable netperf_bridge test on linux

Statically configured NIC bridge clients must be configured via
<policy...> nodes in the config. Otherwise, the bridge can't answer ARP
requests or route traffic.
ldso: fix compiler errors with C++11

The warning

  error: expected unqualified-id before ‘STRING_USERDEF’ token

was caused by a missing whitespace in 'extern "C"int'.