Feature Request: IPv4 only mode for CORE

```
Feature request
What steps will reproduce the problem?
1. We often use IPv4-only XORP custom builds with IPv6 portions stripped out.
Remnants of IPv6 are left in some CORE and config.boot files and are read by
the router. xorp_rtrmgr will refuse to boot until config.boot files are
cleared of all IPv6 artefacts either requiring manual configuration or scripts
to parse and remove superfluous IPv6 addresses.
2. Further, since we only use IPv4, all network laydowns built through the GUI
canvas require additional manual steps to remove superfluous IPv6 addresses.

What is the expected output? What do you see instead?
All config files have either no IPv6 addresses defined or IPv6 addresses hashed
out.

What version of the product are you using? On what operating system?
CORE4.2 on Ubuntu 10.10

Please provide any additional information below.

```

Original issue reported on code.google.com by `phil.wig...@gmail.com` on 9 Feb 2012 at 1:19