Run external script after accepting BGP update or withdraw

[Lauro, John]

> At least, it should make your life easier by handling one route at a
> time without locking. In the first example, parallel execution of such a
> script would lead to a mangled log file.

Actually, on any modern system append mode guarantees not to corrupt from parallel
execution as long you are only adding a single whole line at time, and the file is not on
a NFS mount.