um: vector: Add dynamic tap interfaces and scripting

Provide functionality roughly compatible with the existing qemu
ifup scripting:
* invocation of an ifup script. The interface name is passed as the
  first and only argument
* allocating tap interfaces on the fly if they are not explicitly
  specified

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>