interface: add new "ifup-failed" hotplug event

This hook makes it possible to do some helper work in hotplug scripts
when a connection is not established successfully.

example: try several username/passwords from a pool to establish a
pppoe or wwan connection by replacing the configured values of the
connection in a hotplug script.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>