avoid double-setting of IP address

```
This is to remind of an issue in which both zebra and the daemon try to set the
same IP address; this type of non-fatal error is printed out.

n1: cmd exited with status 512: ['/sbin/ip', 'addr', 'add', '10.0.0.1/16',
'dev', 'eth0']

The suggested solution is to have the code using /sbin/ip first check if the
desired address has already been set on the interface.

```

Original issue reported on code.google.com by `tomh....@gmail.com` on 15 Feb 2013 at 11:36

Start by locating the code that invokes /sbin/ip to add an address, then inspect how zebra and the daemon configure interfaces. Reproduce the duplicate 10.0.0.1/16 assignment if possible; done means an already-configured address is detected before the command runs and the non-fatal error is no longer printed.

Written by the indexing model from the issue text.