netifd: Interface last error support

netifd: Interface last error support

Adds interface last error support which preserves the last reported
error reported by the protocol handler till the interface is up;
e.g. survives network reload and interface restarts.
This is mainly usefull for tracking down why an interface fails
to establish; eg auth failure/traffic limit for PPP interfaces

Protocol handlers register last error support by setting lasterror=1
in the proto_init function

Signed-off-by: Johan Peeters <johan.peeters111@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>