Licencing policy for gdb Python plugins

[John Gilmore]

Since we aren't sure, howabout publishing something that says, "If
your plugin is public domain or licensed under GPLv3+, it's compatible
with GDB's licensing.  If you want to do something different, come
talk with us and we'll work something out."  That way we can make a
decision in a context that's informed by an actual plugin - while
enabling anybody who doesn't mind PD or GPL.

	John