Ensure default route is purged before gateway-defined subint will be UP

ISSUE: Ubuntu affected only.
For vlan/bond enabled interfaces, default route might be created
on start-up time, thus, preventing gateway x.y.z.t would have been
added for default route on its static subinterface(vlan/bond slave) start-up.

F.e. for eth0 auto with dhcp and its eth0.100 subint with gateway x.y.z.t defined:
1) eth0 starts up
2) default route have created for defined by dhcp gateway f.g.h.i
3) eth0.100 starts up
4) gateway x.y.z.t ignored as well ...

SOLUTION: add pre-up hooks for every main/slave interface to erase existing
default route (if any) configured via *this and only* interface to
ensure clean default route would be created despite possible
"dirty" current state of routing table.

Notes:
 -pre-up errors should be ignored to allow int might be UP even
if there is no default route was configured before
 -any alphanum (eth0, bond123, em8 etc.) main interface name should be matched as well
 -if correct interface name cannot be matched from its subint name, use rescue "nullface" value

Fixes: #PRD-2023

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>